International Business Machines Corp. has failed to meet its own forecasts for higher profits in 1985.

While the world's largest computer company yesterday reported that fourth-quarter earnings jumped 23.4 percent, IBM's profits for the full year were down by nearly 0.5 percent. This marked the first time since 1979 that IBM's yearly earnings had declined from the previous year.

The news sent IBM's stock tumbling $5.12 1/2 yesterday to a closing price of $150.87 1/2.

At the beginning of 1985, IBM had projected another record growth year. But an industrywide slump in computers prompted the company to temper its forecasts. However, the company had said that a very strong fourth quarter could make 1985 a more profitable year than 1984.

The fourth quarter proved strong, but not strong enough. IBM said net earnings for the last quarter of 1985 were $2.68 billion, compared with $2.17 billion in the same period of 1984. Earnings per share were $4.36, up 22.8 percent from $3.55 a year earlier.

But for all of 1985, IBM said it had net earnings of $6.56 billion, down 0.4 percent from $6.58 billion in 1984. Earnings per share were $10.67, down 0.9 percent from $10.77 in 1984.

IBM revenue in 1985 rose 9 percent to $50.06 billion from $45.94 billion a year earlier.

Analysts noted that IBM's earlier-than-expected volume shipments of its new 3090 line of mainframe computers were vital to the earnings boost. Some expressed disappointment with the earnings, given that other computer companies such as Digital Equipment Corp. showed stronger-than-expected growth.

"Total orders for 1985 showed good growth over the prior year," said John F. Akers, IBM's president and chief executive officer, in a statement. "Worldwide shipments were higher than in 1984, with substantially all of the growth continuing to be in the non-U.S. portions of the business."

The decline of the dollar in the second half of the year contributed significantly to earnings growth, said analysts.

"The numbers are okay," said Barry Bosak, an analyst with F. Eberstadt & Co. "Currency helped tremendously. It was a difficult year for them, but they did an admirable job. I don't know if there's much helping them for this year."

"IBM has elected to be super-conservative this year," said Ulric Weil, an analyst with the Connecticut-based Gartner Group. "After last year's debacle, they've decided that it would be better to surprise analysts on the upside instead of the downside."