President Carter is considering a new national health insurance plan that would be implemented a step at a time, with each increase in coverage taking place only if health costs were under control at the time.

The plan - or more precisely, a set of alternatives built around the same theme - would eventually cover all Americans. But it would have built-in brakes that could be applied if at any point health cost increases become excessive.

This so-called "triggering in" has been put before the president by his leading health and domestic affairs advisers, it was learned yesterday.

They are also arguing that only by covering all health costs - charges by doctors, hospitals and others - can the government ever hope to bring them under control.

A part of this argument was stated by Health Education and Welfare Secretary Joseph A. Califano Jr. on the television program "Issues and Answers" (ABC, WJLA) yesterday.

"If we do not have national health insurance in this country, we will have the most incredible health cost inflation," Califano said. "The American people will pay more for health care without a national health insurance plan than with it."

"Indeed," Califano said, "health care costs allowed to run wild the way they are running now" will "hit more than $300 billion in 1983" and "make hamburger" out of recent meat price increases. (Beef prices rose a record 6.5 percent in April and 2.5 percent in May, according to the latest figures.)

The idea of combining health insurance "triggering" and braking - and using the whole system to control costs - is intended to help the president meet his recent pledges to advocate "comprehensive" health insurance for all, while mollifying his economic and anti-inflation advisers. They would still like to see him embrace only a few limited additions to the nation's health coverage.

"Triggering," said key administration officials, is the lastest idea of how the nation might achieve broad national health insurance while keeping a hand on the brake in case health cost controls fail.

"For example," one official said, "the president might ask Congress to begin phasing in broadened health insurance in late 1982.

"We would start with insurance for some groups" - say, $20 billion worth of coverage for mothers and children, or for part-time workers who now have poor coverage.

"Then," he continued, "another $20 billion worth of insurance" for someone else might be "triggered" in 1983 or 1984, if general inflation were not above 7 percent or health cost inflation were above 10 percent, just as arbitrary figures. I don't know what the figures might be.

"Or this next step might be triggered if the president or Congress - more likely, the president - simply determined that health cost inflation was within bounds."

"One can think of various combinations of triggers and conditions," another official said. "Of course, we don't know what the president will decide. He may decide against this concept. But I think we'll see some kind of phasing in" - step by step broadening of coverage - with triggering either automatic, if certain conditions apply, or more closely controlled.

Carter's decision may be fully or partly disclosed in a set of "principles" he has promised to give Califano within days. Then Califano can, in his words on television yesterday, "put together a plan."

The president has promised to give Congress this plan in time for Senate health subcommittee hearings late this summer. Organized labor and other health insurance advocates want to try to force congressional candidates on the record this fall, to make the next Congress the "National Health Insurance Congress."

What Congress will do in the next few years is uncertain. The administration has not even been able to get a hospital cost control bill past more than one of four key committees despite 14 months' efforts.

"Still, health costs were about $160 billion last year," an official said. "With the current health cost inflation of 12 percent or slightly higher, they will be $310 billion by 1983.

"So many people are arguing that, technically and politically, the only way to get control is an attractive national health insurance plan with many kinds of controls."