FRANCISCO FIALLOS NAVARRO could certainly pass for a diplomat. He has considerable height, an affable manner, a Harvard degree, a pin-striped suit and a pearl stickpin.

But when he went to present his credentials as the new ambassador from Nicaragua, President Reagan did not treat him like a diplomat. Fiallos might as well have been wearing jungle fatigues and a Fidel Castro button.

Everything went well enough during the Oval Office meeting, with lots of smiles, handshakes and chat about California. But they swapped letters, as is the custom on these occasions. And when the new ambassador read his in the limousine carrying him back to the embassy, it almost burned his hands.

In terms quite unprecedented in the literature of written diplomatic welcomes, Reagan informed Fiallos that Nicaragua is one bad- hat country. He reproached him for Nicaragua's "alarming military buildup," for its enddling in the affairs of its neighbor El Salvador, and he warned its emissary that it better watch its step.

"It was a hard letter," Fiallos said during an interview last week. Then he added, in a philosophical tone that suggests he may do well at his calling, "But really it was just the the same kind of thing as we have had all year from the U.S."

Fiallos was interviewed in the offices of Paul Reichler, a young Spanish-speaking lawyer, who has been representing Nicaragua in the two and a half years since the success of the Sandinista revolution. Relations between the Sandinista government and the U.S. have been stormy. Fiallos is the fourth ambassador who has been sent here to explain that Nicaragua is not a Marxist, terrorist exporter of revolution in Central America, but a democratic, pluralistic nation, two-thirds of whose resources are in private hands.

The green and jumpy leftist government has jailed critics, periodically closed down La Prensa, its leading newspaper, sought Soviet aid, imported Cuban schoolteachers and generally given fits to Washington. The gravest charge made against it by the Reagan administration is that it is a staging area for soldiers and supplies on their way to El Salvador's guerrillas.

Post-Somoza Nicaragua has yet to define itself as a country, even to its own citizens. A current crisis illustrates the point.

Last October, the government imprisoned three business leaders (who were released last week) for publishing a manifesto accusing the junta government of a "Marxist-Leninist genocide plot." It also sent 10 Communists to jail at about the same time for charging them with "selling out to bourgeois-capitalist elements."

In the mind of Secretary of State Alexander Haig, however, there is no doubt whatever about the character of the regime that replaced his fellow West Point alumnus, Anastasio Somoza, the dictator whose family ruled Nicaragua for 45 years.

Haig most recently described Nicaragua as "a totalitarian, militarized state which is subservient to Cuba and Soviet influence."

The secretary of state does not rule out military intervention and threatens a U.S. blockade. To him, the Nicaraguans' real intention to subvert Central America is written clear in its buildup of its armed forces.

The Nicaraguans say they are merely defending their revolution against its enemies, which include former Somoza National Guardsmen who are trained in the United States and make raids out of camps along the Honduran border.

With Fiallos were two high officials of the Nicaraguan government who have come here "to tell our side of the story" to Congress and the press -- Nora Estorga, deputy foreign minister, a bright, direct, 34-year-old Sandinista mother of five, who fought in the revolution until she was six months' pregnant with her first child, and Dr. Ernesto Castillo, the minister of justice. They all deny that arms for El Salvador are being sent through Nicaragua. Their brother revolutionaries receive only moral support from them.

Said Fiallos, "We ask you for proof, we want to see the satellite photographs you say you have. But you say to us that we are not good enough friends for us to share this evidence with us."

Nicaragua has become a flashpoint in the brewing El Salvador debate. "The new Cuba" is being used as a stick to beat more military aid for the Salvadoran junta. Democrats hear that Republicans are going to use "Who lost Nicaragua?" against them in the coming campaign.

Sen. Patrick J. Leahy (D-Vt.) says hardily, "I hope they try it. My mail is running 40 to 1 against further intervention in El Salvador. People who bother to mention Nicaragua say it's none of our business."

Leahy is just back from a flying visit to Managua.

"Everyone I talk to," he reports, "from businessmen to the editors of La Prensa and everyone in between, said one thing to me: 'Don't lock us into becoming a second Cuba.'"

Fiallos' letter to the president, which was not put out by the White House, expressed hope for friendship and respect.