President Carter yesterday signed into law an act that will require all federal employes in grades GS16 and above to disclose in great detail each year their income, assets, debts, any gifts worth more than $250 not from relatives and considerable other financial information.

The new requirements, most of which will take effect May 15, will apply to judges, members of Congress and senior staff members on Capitol Hill as well as to senior civil servants and all political appointees in the executive branch. In all, about 14,000 persons will be covered, most of whom live in the Washington area.

The financial information disclosure law was designed primarily as a guard against conflicts of interest. The information will be kept available for public and news media inspection.

While some disclosure requirements are in force already, it will be the first time so many have had to disclose so much, with criminal penalties to pay if they don't.

The same new law also sets up a mechanism for bringing in Special prosecutors to investigate future allegations that high-ranking members of the executive branch, including the president, have violated criminal laws.

Congress passed the new ethics bill with surprisingly little fanfare or public notice. Most credit Carter and Sen. Abraham A. Ribicoff (D-Conn.), who both supported it; and intervention at critical moments by Speaker of the House Thomas P. (Tip) O'Neill Jr. (D-Mass.) and Senate Majority Leader Robert C. Bryd (D-W.Va.).

The new leadership committed itself to the bill when members got a pay raise last year.

In addition to the disclosure and special prosecutor provisions, the law also tightens rules governing so-called "revolving door" employment of former federal officials. Under the law, a one year "cooling-off period" must elapse before any former federal employe can do business with or before his old agency.

In the executive branch, the disclosure requirements cover all employes rated GS16 or above or earning more than $42,500. That includes virtually everyone in policy-making positions.

Each year, they must file a form with a newly created office in their departments listing all income over $100 and its source. They will not have to give the exact amount, but must specify whether it's between $5,000 and $15,000, $15,000 an d $20,000 and so on.

Anygift they receive from any source, other than a relative, must be disclosed if its value exceeds $250. That includes transportation, lodging, food, entertainment and just plain freebies.

They must list almost anything they own (except their private residence) worth more than $1,000 , including any business interest, real estate or other investment. If they buy or sell an asset during the year, that also has to be disclosed. So must most debts be.

The requirement also generally covers spouses and dependents, except in certain categories.

Those familiar with the act concede that loopholes will undoubtedly be found by those who try. These will depend on the regulations drawn up to implement the bill and the vigor of enforcement.

"But the tools and the framework are there," said Fred Wertheimer, senior vice president of Common Cause, a major lobby behind the bill. Right now, he said, it is "excellent legislation."

The initial impetus for the bill was Watergate. President Carter made it a priority promise during his campaign for the presidency and submitted a proposal of his own in May.

One of the bill's hardest fought provisions was the special prosecutor portion. Under the measure, whenever the Justice Department receives allegation of criminal wrongdoing by any one of a large number of top officials - president, the FBI director, the CIA director and Cabinet members among them - it must filea confidential record on its findings to a special judicial panel.

If the matter is deemed serious, the court must appoint a special prosecutor, who cannot be removed by the president or the Department of Justice.

The special prosecutor provision covers only the executive branch, since it is the Justice Department which finds itself inherently in conflict so often when investigating other executive branch officials.