Billy Graham has sent an urgent letter to his supporters warning that his Minneapolis-based Evangelistic Association is facing a financial emergency and will have to cut back its programs unless it receives more contributions.

The cutbacks already have begun, according to George M. Wilson, executive vice president of the association, who said the association is reducing further the number of radio stations on which the Graham program, "Hour of Decision," is heard. The number, which once totaled 900, is down to 700 and may be slashed to 500, he said.

Also, for the first time, the association is combining the August and Spetember issue of "Decision" magazine, he said.

Wilson said the association's income a year ago did not meet the budget for the first 10 months, but made up for it in the last two months. This year, he said, the association is further behind.

The income so far this year is about even with last year, but costs of postage, paper, printing and radio and television time have increased, Wilson said.

The mail strike in Canada has reduced income from that country, he noted.

Although it has a current budget crunch, the association enjoyed one of its most successful financial years in 1980.

The association and affiliated organizations had combined revenues of $49.9 million in 1980, according to an audited financial statement for the year prepared for mailing to Graham supporters.

This was $11.5 million more than in 1979, a comparison of statements for the two years shows.

Revenues for the organizations last year exceeded expenditures by $1.9 million.

Of the $33.9 million in revenues received in 1980 by the Graham association, $31.1 million came from contributions and most of the rest from estates, deferred gifts and investments. Of the association's $32.6 million in expenditures, $13 million was spent on radio, television and films; $4 million on "Decision" magazine; $3.2 million on international ministries and world emergencies, and $7.5 million on other evangelistic ministries.

In a statement with the financial report, Wilson said he was "pleased" that of the association's 1980 expenditures, only 7.5 percent was expended for fund-raising and 6 percent for administrative costs.