A key Senate critic of President Carter's policy on international arms sales warned the administration yesterday against taking advantage of an accounting change that could raise the promised ceiling on next year's sales by $1.4 billion.

Sen. John Culver (D-Iowa) also called separating the job of formulating arms sales policy within the Defense Department from the job of promoting and administering the sales. At present, one man, Lt. Gen. Howard Fish. holds both.

On May 19, Carter pledged that the dollar volume of arms sales in the current fiscal year would not exceed the total for the year that ended in September.

Until recently, the official estimate for last year's sales was $9.9 billion. But this month DOD announced the estimate had been raised to $11.3 billion by the belated application of a 1976 accounting change that counts add-ons to previously approved sales in the year of the add-on rather than the year of the original sale.

Culver called the accounting change "contrustive." but said its 11th hour application "is the kind of bureaucratic blunder that undermines the confidence of the American public and foreign governments in our national policies."

Since both the Congress and the Executive branch relied on the lower figures in policy discussions in recent months." said Culver in remarks prepared for the Congressional Record, "none of the $1.4 billion increase should be counted by the administration in determining next year's ceiling . . . "

Gen. Fish's two jobs, Culver said, represent "a conflict of interest that should be speedily corrected." He said Carter's arms policy requires DOD to "conduct its own independent and critical review . . . separate and distinct from the program managers and sales promoters . . . "

APentagon spokesman said Fish is being considered for a routine transfer.