After 5 1/2 months of silence, former president Jimmy Carter has let loose a sweeping attack on his successor's policies, criticizing the Reagan administration for "ill-advised" budget cuts, "misguided" environmental actions, efforts to, "smother" the issue of global human rights, and an abandonment of SALT II that has handed the Soviets an "important propaganda advantage."

Carter leveled his attack in a letter to the members of his former Cabinet and senior staff. The document was dated July 3 and mailed from plains, Ga., where the Carters live. Copies were made available to reporters here by Jody Powell, Carter's White House press secretary.

In the letter, Carter criticized at length the budget cuts proposed by the Repubican administration and enacted by the Democratic-controlled House. t

He wrote that "an enormous transfer of government benefits is now taking place from the very poor to the very rich, and middle-income Americans will ultimately have to pay a substantial portion of the cost." He warned of "an inevitable increase in state and local taxes," most likely in the form of "highly regressive property taxes and sales taxes," to reploce lost federal revenues.

"In general," Carter wrote, "budget cuts are always popular with a large number of citizens -- quite often a vocal majority -- in every congressional district throughout our country." But some of President Reagan's cuts, he said, "are on abrupt departure from the commitment of our nation to a better and more productive life for Americans not strong enough or able enough to win these opportunities for themselves. . . .

"Students, farmers, the aged, mentally afflicted and marginally employed Americans will soon begin to suffer personally and in large numbers -- and scientists, educators and those interested in the environment and the arts will come to realize in a few months how much of the vitality of American research and ingenuity and beauty is being quietly vitiated. Only then will the true consequences of some of the new priorities -- specifically, budget cuts and benefit transfers -- become apparent."

Aiming some of his criticism at the Democrats who control the House, at least nominally, he added: "Many of the ill-advised cuts would have been avoided through normal hearings and careful consideration of the best spending options by the congressional committees, but in succumbing so rapidly to strong political pressures a majority of congressmen have let this necessary process be severely and surprisingly short-circuited."

The former president said, however, that these budget fights did not concern him nearly so much as administration decisions in three areas that he spoke of in his farewell address as president. The areas are the environment, human rights and strategic arms limits.

". . . The misguided and radical new policies of the Department of Interior are a serious threat to the future of our nation, condemned almost unanimously . . . for those who are dedicated to the proper stewardship of our nation's natural resources,' Carter wrote. "I have noticed that even the oil companies recognize how foolish are the offshore leasing proposals."

Turning to human rights, he wrote: "There is no way for our government to ignore or to smother this burning issue, or to walk a . . . tightwire between proper torture by 'friendly' dictators and unacceptable torture by others less favored."

Carter said he found it "almost unbelievable" that former Argentine publisher Jacobo Timerman, who says he was tortured in army jails because he is a Jew, has become a subject of attack by an aide to U.N. Ambassador Jeane Kirkpatrick. He added: "Most of us felt more comfortable with the good old days when the American government attacked the tortures and sympathized with the tortured."

Carter said he remains convinced that the second strategic arms limiation agreement (SALT II), negotiated by his administration by criticized by his successor's, remains "highly beneficial." He wrote, "We will surely weaken the Atlantic alliance and severely damage our own reputation as a peace-loving people if we let the Soviet leaders retain the unwarranted but important propaganda asdvantage they have derived from our unwillingness (for the first time since Harry Truman) to seek nuclear arms controls through negotiation."

The former president wrote that he had been urged by members of Congress and by his former advisers to "speak out more forcefully and specifically about our shared concerns, but so far I have been reluctant to do so." He said he felt a president should "have a few months in which to develop and evoke policies and plans with minimal comment or assessment from one's predecessors in the Oval Office."

According to his aides, Carter felt stung by Gerald Ford's somewhat earlier round of attacks, the first of which was made three months after Carter's inauguration, in which Ford said: "Mr. Carter's anti-inflation program came in like a lion. It's going out like a mouse."

Carter chose to make his own first comments in writing rather than in person, and he avoided mentioning the name of Ronald Reagan. And in that manner of self-diminution that became something of an unintended trademark, Carter chose to begin this document of national significance with a few paragraphs of down-home chitchat:

"Except for my mother's recent illness all of us here in Plains are getting along fine, and the prospects for Mother's recovery are very good . . . Billy has a fine job in Alabama and he and his family will soon move to Haleyville. We hate to see them go, but we're proud of his success in a new profession where his talents are being well used.

"Rosalyn and I are working several hours a day writing our books, and still enjoying the work . . . we will continue to spend most of our time in Plains, and will always be glad to have you come by to see us."