In March, Joy Copeland, a 48-year-old Fairfax County woman who worked as a secretary in the Pentagon for five years and has served as a Republican Party precinct chairman, became so concerned about President Reagan's assertions about Soviet missile deployment that she decided to write a letter.

The letter, expressing concern over the possibility of nuclear war and about what she called "our incredible military buildup," was addressed to Soviet leader Yuri Andropov in Moscow.

Yesterday, Joy Copeland's letter, telling Andropov she thought he was right and Reagan wrong in the March argument over missile deployment, was published in the Communist Youth League newspaper, Komsomolskaya Pravda, along with several other letters that the paper cited as evidence of Americans' deep concern about the arms race.

Copeland, who said she spent five years in the Pentagon while a civilian employe of the Air Force from 1954 to 1966, and a member of the Republican Party for about seven years, said she is concerned about what she considers government efforts to portray foreign affairs in black and white terms.

Assertions that "Russians are evil, our plans are perfect--this is what's bothering me," Copeland said.

Copeland described herself yesterday as a Christian.

"One of the things in the Bible . . . is love your enemies . . . care for your enemies," she said in explaining the motivation behind the March 27 letter she said she sent to Andropov.

Copeland said that the reprinted letter is the first of three she has sent to Andropov because of her feeling that he may be demonstrating some indications of an openness that should be encouraged.

"I'm for openness, directness, above all I'm for truth," she said. Copeland, now a student at George Mason University, said she has been reading extensively about Andropov since he came to power, and asked, "Could he be truly trying to reach some kind of peace accord?"

Copeland said she does not trust the Soviets totally, but "if the other guy gives even half an inch," some effort should be made to try to work with them, she said. Expressing the view that Andropov is more open than his predecessors, she called that "terribly significant."

She appeared particularly impressed by the recent letter in which Andropov replied to a Maine girl who had written to him about her concern for peace.

"We are supposed to be the Christian nation," Copeland said. "If he reaches out to any one of us . . . suggesting 'I'm looking for peace' shouldn't we above all say to him . . . All right, we care, we're open, we'll meet you half way."

Copeland said she wrote: "This letter in no way represents an attempt at treason. I love my country dearly, but I love yours also."

She said she wrote in the letter that "I want you to know at least one American feels you're right in your statement that Ronald Reagan is not telling the truth about Russian missile deployment."

She said yesterday that she was referring specifically to the two leaders' March statements about missile deployment.

She said in the letter that she has "learned in recent months to mistrust (Reagan) and his administration from numerous standpoints," including, she wrote, "our incredible military buildup at the expense of Americans and others worldwide for food and other necessities . . . "

The letter also said, "I wish with all of my heart that there was something I could do or say on behalf of all Americans. All I know is that the whole world is holding its breath while you and President Reagan exchange verbal crossfire."

Copeland said she joined the Republican Party in 1974 or 1975 and had served as deputy chairman and as treasurer of the Springfield District. She said she was a chairman of the Woodyard precinct.

She said she quit the Republican Party last year in protest over problems of the needy, and wrote Reagan about it. The Soviet paper said Andropov has received a "steady stream" of mail from Americans agitated about the possibility of war.

James Phelps of Minneapolis, whose letter was also published, said yesterday that "it looks like I am definitely being set up by the Russians." He said, according to United Press International, that some of the quotes were not exactly what he wrote.