WASHINGTON (AP) _ Like most people who go to the hospital, President Reagan will get a big bill after he gets out, but health insurance is expected to cover most of it.

White House spokesman Larry Speakes said Monday that Bethesda Naval Hospital would charge Reagan the standard interagency rate paid by members of Congress and the Cabinet and other government officials.

Reagan underwent a prostate operation at the hospital and is expected to be released in a few days.

Speakes said he didn't know exactly how much Reagan's bill would be, but when the president was hospitalized in August, 1985, and had colon cancer surgery, it came to $423 a day.

The standard charge for military personnel is $3.80 a day.

Speakes said the president's bill will be paid first from the Blue Cross- Blue Shield policy of the California Legislature's retirement system, which he joined when he was governor of California.

''The president will pay any remaining costs from his own pocket,'' Speakes said. ''The president is eligible for Medicare but has elected not to use the Medicare benefits.''

Speakes said he would not disclose what percentage was being paid out of health insurance.

Dale Petroskey, an assistant White House press secretary, said the president is eligible for health insurance programs for federal employees, but chose to retain his California plan instead.