A secret plan being debated hotly in the Pentagon could shift as much as $10 billion from the Navy to the Army to help pay for five light infantry divisions that could be used in mountainous terrain like that separating the Soviet Union and Iran, Defense officials disclosed yesterday.

One reason for making future budget changes that could reduce the Reagan administration's dramatic expansion of the Navy, officials said, is the desire to field Army divisions mobile and hard-hitting enough to confront the Soviets in Iran's Zagros mountains and other rough terrain if they should threaten Persian Gulf oil fields.

Pentagon officials now mapping Army, Navy, Air Force and Marine Corps budgets for the five fiscal years from 1985 through 1989 have concluded they will have far less money than projected under President Reagan's original rearmament program. They believe it will be necessary to shift money from one service to another and to cancel some prized weapons programs.

One Pentagon executive said current discussions of the Defense Resources Board, which is composed of top civilian and military officials, indicate that the Navy will have to give up as much as $10 billion over the next five years to the Army as part of the first serious Pentagon retrenchment since Reagan took office.

Deputy Defense Secretary Paul Thayer has been running the meetings, where the question has been not whether to cut but where. Several officials quoted Thayer as saying, "The Navy must change course. One service is out of our control, and I'm going to get it back."

"Looking back over the last two years, I do feel the Army hasn't shared in the increased budgets to the same extent of the other services," Thayer said in a telephone interview, while declining to comment on what he said in the Pentagon meetings. He confirmed that the Pentagon plans to organize five new light Army divisions at the rate of one a year and that this will take extra money.

Thayer said he has not ordered the Navy or any other service to reduce future budgets to free money for the Army. The decisions on how to apportion funds for FY 1985 through FY 1989 have not been made, he said.

Although the Army's new light divisions would be suitable for fighting in the kind of mountainous terrain that Soviets divisions would have to travel to attack Iranian oil fields, Pentagon officials said they would be capable of going to any trouble spot.

But one military leader said of the planning, "They want divisions for the Central Command," the newly established command that would control U.S. forces sent to the Persian Gulf.

Gen. John A. Wickham Jr., in one of his first interviews since becoming Army chief of staff in June, said yesterday that the new light divisions, which would have 10,000 to 12,000 troops each, will be formed out of existing regular divisions, whose full strength is 18,000.

The light divisions, he said, "would be almost as powerful as a regular division" because of mobility provided by their new helicopters, tanks and troop carriers and firepower to come from new weapons entering the Army's arsenal. The Army has more new weapons on order today than at any time since World War II.

Largely because of the cost of its new weapons, the Army has decided to try to save on personnel. It now has about 779,000 in uniform, or about half its size in 1968, during the height of the Vietnam war.

"The heavy side of the Army is in pretty good shape," Wickham said, elaborating on why he is focusing on building light divisions. "The Soviets have small divisions."

He did not mention any specific area of the world where the new light divisions might fight.

Asked if hard-hitting, highly mobile divisions would resemble the Marine divisions, Wickham said no, declaring, "They're heavy," because of attached units for amphibious operations.

Wickham said the Army receives 23 percent of the defense budget. which is "the smallest it has been in 30 years."

He said he was "uneasy" because "looking downstream, the potential is great" for strategic forces--those for nuclear rather than conventional war--to require so much of the Pentagon budget that there will not be enough left for conventional forces.

"Right now there is a balance," he said.

Asked why the Army fares so poorly when the Pentagon's money pie is divided, Wickham replied, "Part of that is advertising."

He said that every time there is a space shot, the Air Force enjoys national advertising. Every time a carrier battle group is sent to Central America or some other hot spot, he added, the Navy gets advertised to the public in dramatic fashion.

This "translates back into popular attitudes in the United States and probably in the Congress," he concluded.