Cuba plans to demand substantive negotiations toward setting normal relations with the United States as a condition for discussing the departure of nearly 400 Cuban refugees in the U.S. diplomatic mission here, according to informed Cuban sources.

For Cuba, normal relations mean U.S. abandonment of the Guantanamo Naval Base, lifting of the U.S. trade embargo and ending U.S. spy-plane overflights.

President Fidel Castro is aware that this may be among the worst moments in recent years to raise those questions, the sources said. President Carter faces not only a tough reelection campaign, but also crumbling U.S.-Soviet detente and humiliation from the hostage crisis in Iran.

But Castro, they said, believes that such an advantageous opportunity is unlikely again. By allowing the refugee situation inside the U.S. Interests Section here to continue, in essence asking a ransom for the freedom of the Cubans inside, Castro reportedly believes he has a strong bargaining card.

The government has decided "the moment is now," sources said, "because Cuba is rarely in a position to pick a moment." But, they said, Cuba will insist that any agreements reached during possible talks with the Carter administration would also have to be approved by other leading U.S. presidential candidates to ensure that they would be maintained.

A sign of Cuba's seriousness in the conflict with the United States and of its own current domestic turmoil is Castro's decision not to attend Yugoslavian President Tito's funeral.

Although Castro and Tito were strong rivals, attendance at the funeral was a virtual political requirement for Castro, who is president of the movement of nonaligned nations founded by Tito. Significantly, the Cuban delegation to Belgrade also does not include Castro's brother and key aide, Defense Minister Raul Castro.

The three issues Cuba wants to discuss form the basis of its longstanding complaints against the United States. The 1934 lease governing the Guantanamo base gives the United States "perpetual" control over a portion of Cuban territory on the southeast corner of the island. Castro does not recognize the lease and has called repeatedly on the United States to leave.

Carter suspended surveillance overflights of Cuba as a good-will gesture early in his term. But in November 1978 they were temporarily resumed for at least one flight following charges that the Soviet Union sent Mig23 fighters with nuclear capability to Castro. In October, Carter again resumed the flights following his charges that the Soviets had stationed combat troops in Cuba.

By far the most significant of the three issues, however, is the 1960 U.S. trade embargo. Although Latin American nations that participated in the embargo began trading with Cuba in 1975, the United States has shown no intent to resume trade. The United States is Cuba's biggest potential trading partner in the region and a potential source of desperately needed hard currency.

Cubans blame the United States for the worsening relations, which they had hoped would improve under the Carter administration. During Carter's first two years, there was a noticeable improvement.

While Carter suspended the overflights, Cuba agreed to release thousands of political prisoners and let them emigrate to the United States. He permitted nearly 100,000 Cuban exiles to return for visits with their relatives here.

The two countries exchanged cultural and sports missions and opened diplomatic interests sections as an important step to full diplomatic relations.

Then came a number of controversies, beginning with U.S. unhappiness at the presence of thousands of Cuban troops in Africa, U.S. charges that Cuba participated in the invasion of Zaire's Shaba Province in 1978, the Mig23s, and most recently the Soviet combat troops on the island.

But Cuban officials say that, despite the bitterness, they were anxious to keep open diplomatic channels and sent several messages of good will to Washington.

One example the Cubans cite is a bilateral antihijacking agreement, which Cuba recently allowed to lapse, although Havana said it would respect the spirit of the accord.

On several occasions in recent months, Cubans have hijacked boats to flee to Miami, "where they are received as heroes," one Cuban official said. But, officials here point out, Cuba has prosecuted all but one of the Americans who hijacked planes to Cuba in the past year.

Thus, the current refugee crisis comes in a climate of mutual suspicion. Castro is known to be infuriated at the versions Carter and the U.S. press have given of violence at the U.S. mission last week. These accounts entirely blame Cuba and describe the incident, in which approximately 800 persons waiting outside the mission for visas fought with Castro supporters, as a premeditated assault by government security agents.

In recent days, Castro has insisted privately that the clash, which caused nearly 400 of those waiting for visas to run inside the U.S. mission building and refuse to leave, began as militant Castro supporters and opponents insulted each other and then beat each other up with no instigation from the government.

While the 387 Cubans have now spent their sixth day in the American mission, Cuba apparently has decided to focus its attention on them and has allowed the diplomatic impasse to speed up refugee departures by boat.

An average of 3,000 people a day has been leaving this week, three times the rate of the week before. Yesterday, 4,033 left from the port of Mariel, according to the government newspaper Granma, bringing the total to nearly 22,000 since castro opened the sea bridge to southern Florida two weeks ago. Granma said an additional 1,404 U.S. boats were still waiting to pick up refugees.

Cuban officials openly admit, often with a vengeance, that many people with police records have been encouraged to leave for the United States.

Police work their way down lists of those with criminal records, show up at people's homes and even inside the prisons, it is reported here.

[At the request of the State Department, the Civil Aeronautics Board rescinded approval for tourist flights to Havana from Washington's Dulles International Airport. The flights, aboard Cubana Airlines, were to have been inaugurated yesterday.]