Denying military people their pay raise this year will "almost certainly" hurt morale and combat readiness, Marine Commandant Robert H. Barrow warned Defense Secretary Caspar W. Weinberger on the same day that the Pentagon issued a statement in Barrow's name that "service people are willing to accept their share of the sacrifice."

Barrow was acting chairman of the Joint Chiefs of Staff, and in both cases was speaking for all four of the service leaders.

Pentagon officials, who declined to be quoted by name, said yesterday that the two seemingly contradictory statements were really not in conflict because they were dictated by different circumstances.

Whatever the explanation, Barrow's private warning to Weinberger hands ammunition to critics of President Reagan's decision to freeze military pay rather than cancel weapons to reduce Pentagon spending in fiscal 1984 by $8 billion.

"The fact is that a pay freeze, resulting from failure to provide the fiscal 1984 pay adjustment, following a 4-percent pay cap in fiscal 1983, almost certainly will have an adverse impact on personnel retention and, as recent history has dramatically shown, a long-term negative effect on combat readiness of U.S. military forces," Barrow said in his private memo to Weinberger.

"It is unfortunate," Barrow continued, "that such a decision became necessary at a time when the military services were beginning to see a turnaround in previously poor retention rates."

Barrow's memo was disclosed first in the privately owned Army Times newspaper, which has a wide circulation among the professional military people the Reagan administration is trying to keep in the all-volunteer force.

Barrow wrote Weinberger that he, as acting chairman of the Joint Chiefs, was sending the memorandum on behalf of all four service leaders.

"Our immediate concern is that the president's comments made in Dallas on 11 January" that forgoing the pay raise " 'is not setting us back in any substantive way at all in our defense program' may be interpreted as demonstrating a lack of sensitivity to the needs of the members of the armed services."

Barrow appended to his Jan. 13 memo a statement that the chiefs wanted Reagan to put in his State of the Union message, including a pledge not to tinker with the military retirement pay, health-care programs or commissary benefits "we have traditionally provided our military people."

But Reagan, aside from saying he was "sorry" about having to freeze military pay, did not in his message put military retirement or other service benefits off limits in the search for cuts, as the chiefs had recommended.

Although the chiefs struck out with Reagan, Weinberger on Wednesday issued a statement to military people saying, "I will strongly urge that the 1985 budget provide for the full recovery of the pay cap in 1984." This October's military pay raise had been projected at 7.6 percent to keep service people abreast of inflation.

The first public evidence that the Joint Chiefs of Staff were unhappy about the pay freeze came on the morning of Jan. 13, when Gen. Charles Gabriel, Air Force chief of staff, told reporters in response to questions at a breakfast meeting that the chiefs had not been consulted and would rather have achieved the reductions through stretchouts or cancellations in the hardware accounts.

Gabriel further disclosed that he had not been informed of the president's decision to freeze pay.

That afternoon, the chiefs drafted a statement in hopes of offsetting some of the stories and broadcasts that followed the Gabriel breakfast and which, according to Pentagon officials, the chiefs felt portrayed them as being in open rebellion against their commander-in-chief, the president.

The fact that the statement was expressed in the third person, instead of in Barrow's own words, and was issued by the Pentagon press office added to the impression that the chiefs had been forced by their civilian superiors into making the statement to take the political sting out of Gabriel's remarks.

Barrow's separate memo to Weinberger was prompted by Reagan's remarks on the pay freeze two days earlier in Dallas and was not an attempt to challenge the president's final decisions on the issue, Pentagon officials said yesterday.