Another strong surge in hiring last month pushed the nation's unemployment rate down slightly to 5.8 percent, the Labor Department reported yesterday, giving fresh evidence that the economy is not slowing down.

The rate has been either 5.8 percent or 5.9 percent, as it was in December, for the past six months. It has not fallen despite the fact that nearly 1.6 million additional persons found jobs over that span because the work force has grown just as fast.

So many persons are now at work, Janet Norwood, acting commissioner of the Bureau of Labor Statistics, told the congressional Joint Economic Committee, that the proportion of people in the working age population who have jobs is at an all-time high, 59.3 percent.

The jobless rates for adult men (4 percent), adult women (5.7 percent), and teenagers (15.7 percent) were little changed from December.

Similarly, the rates for whites (5.1 percent) and for blacks (11.2 percent) are still just about at the levels reached last summer. Over the past year, the rate for blacks has fallen from 12.8 percent, while that for whites has only edged down from 5.5 percent. However, Norwood noted, "in January 1979, the rate for blacks was still more than twice that of whites."

Unemployment for black teenagers dropped to 32.7 percent, except for last August, the lowest since 1973. There were 341,000 black teenagers unable to find jobs last month, compared to 393,000 a year earlier.

Some economists, surprised at the continuing robustness in the economy, are even beginning to wonder if the administration might have to take additional steps to cool things off a bit.

"This isn't at all what we expected to happen," said economist George Perry of the Brookings Institution, "but you've got to take it for real."

Perry added, "It really does make you ask, is monetary policy biting at all?"

The monthly household survey showed an employment gain of 450,000 in January. Actual payrolls data from reporting companies showed a 325,000 increase, with the new jobs concentrated in the service-producing sector, particularly in retail trade. Manufacturing payrolls also rose by 65,000, however.

As a further measure of the improvement in the past year, only 523,000 persons unemployed last month had been without jobs for more than six months. In January a year ago there were 803,000 persons in that category.

One-half of the 3,033,000 jobless last month had been unemployed for less than six weeks.

Norwood told the JEC, "An unusually large number of women entered or re-entered the civilian labor force in the past year, and by January 1979, 43 million were in the labor force... By mid-1978, one out of every two women 16 years old and over was working or looking for work, and the proportion has edged up further in recent months.

"Most of the recent labor force gains, like most of those throughout the 1970's occurred among women under 35," she continued. "Despite the pressures of combining a job with of women 25 to 34 years old -- 71 percent of whom are mothers with dependent children in the home -- continued to enter or re-enter the work force."

Norwood also said that the number of families with more than one wage earner has risen dramatically. "By March, 1978, 27.5 million husband-wife families had more than one earner," which was 58 percent of all husbandwife families.

The increase in employment last month does not imply quite such an increase in production, according to Norwood, because the index of aggregate weekly hours, which reflects both employment and the length of the average workweek, edged down. The length of the workweek dropped from 35.9 hours to 35.7 hours.

The administration has forecast a gradual increase in unemployment to 6.2 percent as the economy slows down in the second half of the year. Should the economy remain so strong that the rate declined instead of rising, observers expect an attempt to tighten monetary policy so as to put a crimp in new housing construction.

Other forecasters still expect a recession that would send unemployment much higher, to 7 percent or more.