The Carter administration plans to replace about half ot the 120 U.S. ambassadors around the world within the next six months by means of a highly unusual and still-evolving selection process, informed officials said yesterday.

President Carter's new Advisory Board on Ambassadorial Appointments, which brings outsiders into the deliberations over envoys for teh first time, is already screening the names of potential appointees to more than 15 key ambassadorships, including London, Paris and Tokyo. The 20-member group, which was announced Thursday, ended its initial two-day meeting at the State Department yesterday noon without completing its recommendations for any of the ambassadorships.

Carter told the board in a White House meeting Friday that in a general "a few too many" ambassadorships are now held by noncareer appointees, participants said. At present, about one-third of the U.S. ambassadors worldwide are political appointees.

Many of the ambassadors who will be left in place after the first months of the new administration will be those in relatively obscure countries, posts now held by career Foreign Service officers. Major ambassadorships, particularly those held by political appointees, are traditionally rotated by an incoming President.

In the same meeting, Carter told his new board that "you can start with a clean slate" because he has given no promises to anyone for a diplomatic post except for the nominations already announced for envoys to Italy and Lebanon. He also told the group he does not want personal finances to be a consideration in the selection of ambassadors, a policy which may require an infusion of additional entertainment money for some European posts traditionally held by affluent appointees.

"We are plowing new ground," Florida Gov. Reubin Askew, the board chairman, said in an interview after yesterday's session. Askew and Deputy Under Secretary of State-designate Richard Moose explained how the new board evolved and how its role is already expanding.

Before Inauguration day, Carter decided to solicit recommendations for ambassadorial nominations from 14 people, including women, blacks and Hispanics, spread around the country. After the names began coming in, Carter and Secretary of State Cyrus R. Vance broadened the concept of the outsiders' role to include the screening of noncareer appointees proposed from any source, according to this account.

The 14 people originally asked for recommendations were augmented by six more to form the new advisory board. With the exception of former Ambassador Averall Harriman and former U.N. Ambassador William Scranton, most members of the board have not had diplomatic experience. Many of the board members however, were early supporters of Carter's candidacy for President. There are eight women, three Hispanics and at least two blacks.

The originally plan for the group was to study a list of two to seven and names for each ambassadorial post supplied by high State Department officials, and eliminate the names of any prospective political appointees who were deemed unqualified. The group was also to be given names and background sketches of career diplomats being considered, but was not expected to study them in detail.

Under the procedure adopted in its initial meetings, however, the board of all potential appointees - career or noncareer - that are supplied for each post by the State Department. It will also feel free to add suggested nominees for any post from a list of all persons whose names have been proposed by its own members or anyone else, Askew indicated.

A list of those who have been screened and found qualified by the committee for each post will be given to Carter, who will make the selection. Askew said he expects to present the board's findings to Carter in person in many cases.

Askew said that, at Carter's request, he had made "a couple" of suggestions himself of people to be considered for ambassadorships. This was before undertaking the job of chairman of the review panel, he said. The Associated Press reported that Dr. Claud Anderson, Askew's education adviser, and Tallahassee lawyer James Smith, a former Askew aide, are among those who are seeking posts as envoys to the Bahamas and Spain, respectively.

Askew said "there is a clear understanding that there will be no advocacy by any member of the board" of any candidate for ambassador, particularly in view of their dual role as judges and proposers.