The Reagan administration, convinced that Cuba is masterminding leftist guerilla efforts to take over El Salvador by force, is preparing to give the Salvadoran government increased military and economic aid that will make that country the first testing ground of President Reagan's campaign to combat communist-inspired revolution.

Recommendations on aid and other actions that would greatly increase U.S. involvement in the bloody civil war in El Salvador are being prepared for presidential approval, and the administration tentatively plans to reveal them next week in a major policy statement, possibly to be delivered by Secretary of State Alexander M. Haig Jr.

If the extensive interagency policy review being conducted under the leadership of the State Department can not be completed by then, announcement of the administration's full range of decisions is likely to come during the week beginning Feb. 23, with perhaps a "white paper" outlining Cuban involvement in El Salvador being published next week.

When the announcement is made, it is certain to produce a storm of protest in this country from a broad spectrum of groups ranging from congressional liberals to the Roman Catholic Church. Their objections center on doubts that the Salvadoran leftists are communists under the influence of Cuba, the contention that the government there is a captive of ultra-rightist forces that have engaged in wholesale murder and violations of human rights, and a concern that the United States is heading for a Vietnam-like involvement in Central America. If the United States sends El Salvador more military equipment, it also will have to send more technicians and instructors to see to the equipment's use.

Nevertheless, sources said, the administration is determined to press ahead with its El Salvador policy, partly as a means of driving home to the Soviet Union and its communist-bloc allies, such as Cuban President Fidel Castro, that this country will back up with action its rhetoric against exploiting revolutionary situations around the world. That line, which is at once the central and most controversial aspect of Reagan's foreign policy, was laid down vividly by Haig at a Jan. 28 press conference, where he said that "international terrorism will take the place of human rights" on the list of U.S. concerns.

To buttress its arguments, the administration is known to have what it considers hard and convincing evidence -- collected partly during the final weeks of the Carter administration -- documenting Soviet and Cuban support for the Salvadoran leftists. Sources said that this evidence already had convinced most policymakers in the Carter administration that Cuba was sending a greatly increased flow of arms to the Salvadoran guerillas through neighboring Nicaragua, whose leftist-dominated government is sympathetic to the insurgents.

Part of the evidence consists of documents captured by the Salvadoran military from the guerrillas, containing what appear to be pledges of arms support from various communist governments. U.S. officials confirmed last week that such documents existed, but said they could not vouch for their authenticity.

The clinching evidence, sources said, has come from intelligence reports that U.S. agencies began to receive early in December. Previously, the State Department had insisted it had no evidence to support charges by conservatives that arms were flowing into El Salvador from Nicaragua.

But beginning about two months ago, according to well-placed officials, large numbers of Soviet-built Cuban transport planes began shuttling into the Nicaraguan capital of Managua during periods when the airport was closed to other traffic, and unloading their cargos under conditions of secrecy.

Subsequently, the sources said, two planes that had come from Nicargua and apparently been used to ferry arms were disabled in El Salvador. In one case the pilot was captured and authorities were able to question him. In addition, the sources added, at least one truck loaded with rifles and known to have come from Nicaragua was captured in neighboring Honduras while trying to cross that country's border with El Salvador.

U.S. officials are known to believe on the basis of all these fragments that there was an accelerated supply effort by Salvadoran leftists' outside supporters in preparation for the large but ultimately unsuccessful offensive mounted by the guerrillas last month.

Although the reliability of this evidence is certain to be questioned by opponents of the Salvadoran government, U.S. officials believe it is hard to ignore or refute, and the sources said the administration intends to send a team of officials to West Europe this weekend to present this evidence to governments there. The purpose will be to counter campaigns in some European countries, notably West Germany and Sweden, to pressure the United States into dropping its support for the civilian-military coalition ruling El Salvador.

One team headed by Lawrence S. Eagleburger, who is to be assistant secretary for European affairs, will leave tomorrow to visit France, West Germany, the Netherlands and Belgium. Two other teams will visit various other countries in what one source called "a full-court press."

Of even greater import, the sources said, this recent intelligence has been the principal factor in causing the administration to decide that it has to draw a line making clear that it will not allow a leftist takeover of El Salvador by military means and that it will meet every attempt to escalate the conflict with a corresponding increase in U.S. assistance for the government's counterinsurgency efforts.

Essentially, the policy being carved out by the administration means continuing and deepening the support given by former president Carter to the ruling coalition, which claims to be seeking a centrist position between El Salvador's feuding leftists and rightists. Although the Carter administration's support faltered after four American Roman Catholic women missionaries were murdered with the suspected complicity of government security forces, Reagan and Haig have put the United States squarely back in the government's corner.

Despite charges by the coalition's foes that it has no popular support, U.S. officials are encouraged that the failure of the guerrilla offensive last month means that the leftists also do not have the confidence of Salvador's 4.3 million people and that, given time, the government can win sufficient backing to bring the civil war under control.

Although the specific details of increased aid have not yet been decided, sources said they almost certainly will include a jump in military assistance from the current $10 million level to approximately $25 million to $30 million over two years. Most of this would go to providing El Salvador's 8,000-man army with such staples of counterinsurgency warfare as helicopters, small arms and ammunition.

This would require increasing the number of U.S. military training and maintenance personnel -- currently running about 20 persons -- in El Salvador. The sources said the administration, sensitive to the comparisons that might cause with the early days of U.S. involvement in Vietnam, does not plan to send any combat advisers there at this point.

In economic aid, the administration is readying a package of measures to help the Salvadoran government grapple with immediate financial problems and also pursue its agrarian reform program and other actions designed to win support from the Massachusetts-sized country's largely impoverished peasantry. In his fiscal 1982 aid request to Congress, Carter asked for $40 million for El Salvador; sources said that Reagan, despite his desire for budget cuts, could raise this request to as much as $100 million.

The sources added that, in a parallel move, the administration is continuing its pressure on Nicaragua to choke off the arms flow, using as a lever the threat of ending the U.S. financial aid that Nicaragua badly needs to help stave off fiscal chaos.

According to the sources, the first signs are that the Nicaraguans, while maintaining publicly that there is no arms flow, are trying to comply. But, these sources stressed, it is still too early to say whether the Nicaraguans will act with sufficient swiftness and effectiveness to prevent Reagan from losing patience and taking more drastic action against that country.