For the first time, the World Bank plans to borrow a portion of its money needs at floating, or variable, rates in short-term markets and to lend money to member nations at a rate that will change every six months instead of being fixed at the time of the lending commitment.

These significant changes in bank borrowing and lending practices are expected to be announced after a final executive board discussion Thursday. Under the proposal by its management, the bank would borrow up to $1.5 billion at variable rates in the fiscal year starting July 1 out of a total need of about $9 billion.

The precise form of paper or certificate that the bank would sell, and to which precise short-term rate it would relate, have not been decided.

One basic rationale behind the new plan is to put the bank on a better footing, in which its overall returns on its loans bear a closer relationship to its true costs of borrowing money. In a rising-interest-rate market, the bank bears most of the risk: It now has about $30 billion of loan commitments outstanding, yielding about 8 1/2 percent. But the bank in the past year had to pay an average of 11 percent for new money.

Another reason for the proposed change is an expectation at the bank that interest rates eventually will decline from present high levels. If that happens, both the bank and its customers can benefit by not being completely locked into fixed, long-term commitments.

Permission necessary for the bank to borrow in the U.S. short-term money market already has been received from Treasury Secretary Donald T. Regan. Permission is also expected from West Germany and will be sought from other key countries. The bank also expects to be tapping the short-term Eurodollar market as a source of funds.

Normally, the bank raises money by floating bond issues at fixed rates in major capital markets. Most of the bank's money still would be raised in that fashion. But by being able to borrow some short-term money at variable rates, the bank would gain flexibility in building the overall pool of money it needs for lending to member nations.

The new variable-rate lending procedures would apply only to regular loans by the bank, not to those made by the International Development Association, the bank's low-rate lending agency.

The decision to borrow short-term money recognizes that the bank's need for funds is growing at a time when the availability of long-term money is limited and interest rates are near a peak. Officials believe that at the moment, although they might have to pay 16 percent or more in the U.S. market for seven-year money, they could borrow on a shorter basis here for 12 to 13 percent.

World Bank officials are sensitive to the possible criticism that they are falling into the trap of "borrowing short and lending long"--a potentially dangerous banking practice that has seriously hurt many institutions, among them U.S. savings and loan associations. But the World Bank's plan is to establish a lending rate based on the cost of the pool of funds built through both variable-rate and fixed-rate borrowings.

The World Bank's lending rate--now about 11.6 percent after two boosts in the past year from 9.6 percent--would vary every six months based on the cost of money in the pool, plus one-half percentage point. As proposed by the bank management to the board, borrowing countries would not have the option to borrow at a fixed rate for 15 or 20 years, as they do now. They would have to borrow on the variable-rate formula.