UNITED NATIONS (AP) _ President Reagan praised U.N. budget reforms that give industrialized nations a larger say in allocations, and a U.N. spokesman quoted him as promising to seek an end to cuts in U.S. contributions to the world body.

In a statement released Wednesday, Reagan said the U.N. General Assembly ''took an historic step to adopt sweeping reforms of its organization and methods of operation. We are very pleased that these measures have finally been adopted.''

U.N. spokesman Francois Giuliani said Wednesday that Reagan telephoned U.N. Secretary-General Javier Perez de Cuellar on Dec. 22 to express satisfaction with the reforms.

Giuliani quoted the president as saying he ''hoped it would enable him to go to Congress and obtain the necessary funds.''

He quoted Perez de Cuellar as replying: ''I am counting on the efforts of the administration to obtain the necessary funds for the United Nations.''

The United Nations assessed the United States $210 million in 1986 dues. But Congress, contending that the world body spends too much and is poorly managed, authorized only $100 million.

The U.S. action contributed to what Perez de Cuellar called the worst financial crisis in U.N. history.

In late December, the General Assembly approved reform measures that give the industrialized nations, which contribute most of the body's funds, a greater say in deciding the budget. In the past they routinely have been out- voted by developing nations, which form a majority but give little money.

The reform package says the key Committee for Program and Coordination must reach decisions by consensus. That essentially gives each of the 21 committee members a veto.

Even now, U.N. sources do not expect the United States to restore all the $110 million cut in 1986. But the sources, who spoke on condition of anonymity, said they hoped Reagan would ask Congress for a supplemental appropriation of $42 million for 1986 and the full assessed contribution of $212 for 1987.

The United States is the largest U.N. donor, assessed 25 percent of the world body's costs.

Reagan said in his statement: ''I am particularly gratified that the reforms included agreement to use consensus in deciding on budget levels and program priorities. This new procedure will assure broad-based support for the vital activities of the United Nations around the world.''

The United Nations also is trimming its staff by 15 percent, including a 25 percent cut in the top-heavy executive echelons.

The money withheld by the United States left the United Nations, which has an annual budget of about $800 million, nearly broke at the end of 1986. Perez de Cuellar said it ended the year with $10 million, enough for only a few days' operation.