The Army will not be able to achieve its goals of catching up with the Soviet Union by 1985 or surpassing it by 1990 under the new budget cleared in a series of scrub-down sessions inside the Pentagon, according to a secret document obtained yesterday.

The budget document, which the Defense Department confirmed as authentic, warns that cuts in the fiscal 1982 Army budget not only slow the planned development and production of weapons but make it difficult to fill the ranks with the high quality soldiers Congress is insisting upon.

The Army staff sounded these and other warnings in an "executive summary" of what impact this year's decisions would have on the Army's ability to carry out national strategy.

The document, stamped secret, is the latest in a series of security leaks that seem especially prevalent in this year's election contest where national defense has become a major issue.

Pentagon spokesman Thomas B. Ross, when asked to comment on the complaint that the administration is giving too little to the Army to fulfill commitments, said the Army summary "is obviously designed to get the Army as large a share as possible of the total Department of Defense budget."

Defense Secretary Harold Brown has not yet seen the Army assessment, Ross said. It was addressed to him, but has been shortstopped by the Pentagon's controller office for study.

When Brown does get the Army summary, said Ross, it will be "carefully studied to make sure that enough funds are requested for the Army and other services to meet our national security needs."

Although Ross sought to downplay the Army assessment as "as a routine part of the annual pull-and-tug of the Defense Department budget process," its findings hand ammunition to the campaign of Ronald Reagan, the Republican standard-bearer who already has been firing away at President Carter's defense policies.

Here are major findings in the 18-page "executive summary" written by the Army staff and sent to Brown in late September after a series of decisions was made on the new defense budget that will go to Congress in January:

Under this "constrained resource level" of $47.3 billion for the fiscal 1982 basic budget, "the Army will be unable to achieve either the goal of qualitative equivalence with the Soviets by 1985 or the goal of technological superiority by 1990."

Pentagon civilians reshaped the Army budget to the point that the planned modernization of its arsenal lost out to the drive for "near-term readiness," putting weapons already in hand in fighting condition." The result is an unbalancing of the Army's long-standing research and procurement programs for the 1980s."

Unless the balance between developing new weapons and fixing up old ones is restored, the Army warned, the service will fall "further behind the U.S.S.R. both qualitatively and quantitatively."

To buy the guns, tanks, helicopters and other weaponry the Army figures it needs to close its gap with the Soviets, $122 billion would have to be spent in fiscal 1982 through 1986, according to the secret analysis. But the administration plans to provide only $68 billion -- $54 billion less than the Army staff claims it needs.

"The Army has severe equipment shortages today," the summary said, citing as "an example of our distress" the fact that 50 of 155 forward deployed support units in Europe are rated C-4, a low state of readiness.

The Pentagon's plan to reduce the civilian work force on its payroll from a planned 364,000 for the Army in fiscal 1981 to 355,000 in fiscal 1982 will make it harder than ever to field a quality force, according to the assessment. mOn top of cutting the civilian force, the Army complained, it is under orders to convert 20,000 military billets to civilian ones.

Because of that combination, "the Army has lost the capability to pursue sound management solutions to pressing total manpower problems."

The Army "underestimated" the difficulty of attracting male high school graduates to its ranks, falling, 14,000 short of the goal of 80,000 in fiscal 1980. "The decline in the high school graduate population has commenced," the Army stated. "Recruiting will be much more difficult and additional resources are required" beyond those provided in the new budget.

The Army also warned that it does not have enough money to help defense contractors gear up in a hurry to fill military orders if the country goes to war. This would take something like an extra $3.8 billion, it said, about two-thirds of which would be far ammunition alone.

Under the proposed funding, the Army said, it would not have enough ammunition and other supplies to sustain forces in a war in either the NATO sector or in South Korea for as long as current war plans require.