The conservative Heritage Foundation, which supports President Reagan's defense proposals, says money the President obtained to strengthen the US military is being ''soaked up'' by existing weapons programs whose costs are increasing more rapidly than planned, according to a Wall Street Journal report.
Consequently, the study asserts, ''The Reagan administration has put together impressive budget resources . . . with results sadly below expectations.''
The analysis, which has been privately presented to top White House officials in recent weeks and is to be made public this week, contends that cutting the defense budget will only make things worse. It also concedes that faulty Pentagon weapons-buying became a problem long before the current administration.
But the study, directed by George Kuhn, a military consultant to the foundation, flatly contradicts Defense Secretary Caspar Weinberger's recent statements that the practice of underpricing weapons is a thing of the past and that the Reagan buildup has already produced big gains in US military strength.
The foundation calls on the Pentagon to save the buildup by canceling some high-technology weapons of ''marginal utility'' and using the money to build inventories of other weapons, bolster readiness, and add more troops - an approach the Pentagon has resisted.
The study analyzes 28 major weapon systems using official Pentagon cost estimates.