President Reagan ordered a 15-month pay freeze yesterday for the government's 1.4 million civil servants.

In doing so, he rejected a report that said an average raise of 19.1 percent would be needed to match salaries paid workers doing comparable jobs in private industry, and also rejected his own original proposal earlier this year to cut federal salaries 5 percent next year.

White House spokesman Larry Speakes, in Santa Barbara where the president is vacationing, said the 19.1 percent raise, proposed by federal pay agents, would have added $20.6 billion to the budget, resulting in an increased deficit that could harm economic recovery.

Military personnel, who usually receive the same raise as federal civilians, would be given a 3 percent raise in October under legislation that has cleared the Senate and is before the House. The next raise for civil servants would come in January 1987. Congressional budget estimates project that raise at about 3.8 percent.

Reagan's recommendation has been anticipated since the Senate and House included a pay freeze in the compromise budget they adopted before recessing for the summer. Many congressional observers and federal union leaders maintain that the White House threat to cut federal pay -- or fire up to 125,000 workers to cut costs -- was really a ploy to win congressional support for a freeze.

Kenneth T. Blaylock, president of the American Federation of Government Employees, said the pay-comparability law has "become a sham." He said it is time to find an alternative "and perhaps consider" setting federal salaries by comparing "total compensation" (wages and fringe benefits) in government and industry.

Reagan made his proposal after receiving a report from his pay agents: Labor Secretary William E. Brock, Office of Management and Budget Acting Director Joseph R. Wright and Office of Personnel Management Director Constance J. Horner. The report is required by the Federal Pay Comparability Act of 1970.

That law says the president's pay agents must make a recommendation for annual October salary adjustments based on private-sector wage data collected by the Bureau of Labor Statistics. But it permits the president to submit an alternate plan -- no later than Aug. 31 of each year -- if he wants to raise, lower or delay raises.

If the alternate pay plan had not been submitted, the 19.1 percent raise proposed by the pay agents would have gone into effect in October, the start of the new fiscal year. The freeze takes effect unless Congress acts to raise salaries.

Presidents Richard M. Nixon, Gerald R. Ford, Jimmy Carter and Reagan all have used the alternate report system to lower or delay proposed federal raises. They usually cited economic grounds, and also said that the survey did not include enough firms, or kinds of jobs in industry, to present a true picture of private-sector pay.

The last three presidents proposed that the survey be expanded to include salaries paid 12 million state and local government workers, and that the comparisons also take in the value of fringe benefits offered in industry and government.

Last year the pay agents' report said it would take a 21 percent raise for the government to catch up with industry. Instead, Reagan recommended a 3 1/2 percent raise that went into effect last January. A year earlier, white-collar federal workers got a 4 percent raise.

According to the Office of Personnel Management, the average white-collar federal worker earns $25,247. The Labor Department reported this month that the average pay of 93 million private-sector workers surveyed was $18,350.