The Soviet Union's economy continued its steady, expansion in 1977 - but at a slower rate - statistics released tonight by then National Central Statistical Board indicate.

These figures show that industrial output of this nation's vast, planned economy rose 5.7 percent in 1977 over the output for the previous year. This was a rise that was just one-tenth of 1 percent more than called for in last year's plan.

This and other statistics released by the planners indicate that the slight stagnation of economic growth that set in here a decade ago in continuing. Just last month, the leadership announced production and economic goals for 1978 that are measurably lower than those called for in the current five-year plan. These figures indicate that it will be increasingly difficult for the Soviets to fulfill the general goals of the 1976-80 five-year plan, which was envisaged as recently as a year ago as a blueprint for narrowing the economic gap between the United States and the Soviet Union by the end of this decade.

In part, the production results for 1977 also indicate that the Soviets, despite their repeated scorn of the economic ups and downs of capitalist countries, are themselves not immune to the vicissitudes of the world economy. While many Western authorities consider the Soviet economy enormously deep and resilient, such unexpected problems as a multimillion-ton shortfall in the 1977 harvest have impact on overall economic expansion.

The figures released by the Soviets, which cannot be verified by Western observers, say that individual productivity rose 4.1 per cent in 1977 over the previous year. And that per capita income increased by 3.5 per cent.

Productivity is an important measure of increasing efficiency in a national economy which the Soviets themselves admit is plagued with inefficiencies and chronic shortcomings.

The government said that the nation produced more varieties of goods than ever before and that most areas of the ecomony exeeded their norms for the year. Two hundred and fifty major new industrial enterprises were launched last year, according to the Kremlin, an indication that the nation's productivity continues to expand at a healthy rate.

The government placed the chemical and petrochemical industries at the top of a list of rapidly developing sectors. The nation earns hard currency by selling its plentiful oil abroad. Nevertheless, its trade with the West in the items the planners want in order to streamline and improve production - computers and related technological innovatins - has suffered from the poor 1977 harvest. The 19 million ton shortfall forced in Sovieets to spend hard currency for grain instead of machinery and other equipment, thus somewhat blunting the push to improve economic productivity.