The United States' merchandise trade deficit declined in October from its record level of September, although it continued to head toward a new high for the year, the Commerce Department reported yesterday.

The October deficit totaled $11.5 billion, close to the monthly average this year but far lower than the September figure, initially reported as $15.5 billion.

Nonetheless, the deficit for the first 10 months totaled $118.1 billion, and it is expected to approach $150 billion at year-end, overwhelming the record of $123.3 billion, set last year.

Commerce Secretary Malcolm Baldrige called for "greater efforts" from Congress and industry to lower the trade deficit, which has held back the country's economic growth during the year and hit hardest at America's manufacturing might.

Plant closings and the loss of 270,000 manufacturing jobs this year because of the trade deficit, moreover, have led to fierce protectionist pressures in Congress, forcing the Reagan administration to adopt a more aggressive policy against the overvalued dollar -- which is blamed for at least half the trade imbalance -- and against other countries' unfair trade tactics. The high dollar and unfair trade tactics hurt U.S. sales overseas and help foreigners sell goods to this country.

"Congress should adopt a plan to reduce federal budget deficits and bring the dollar down further. Business must modernize their facilities to compete in international markets," Baldrige said.

Alexander Trowbridge, president of the National Association of Manufacturers, said the trade deficit in manufactured goods, an area in which the United States maintained a surplus as recently as 1981, is responsible for three-fourths of the total deficit this year.

"The ability of American firms to compete has been seriously undercut by an exchange-rate system that has not been working properly and a general failure of U.S. policy to give commercial competitiveness the priority it deserves," he said.

Although the dollar has fallen about 20 percent since February, economists said the decline is unlikely to be reflected in U.S. trade figures for another six months or more.

"It is far too soon for the decline in the dollar to have any effect on the deficit," said Larry Chimerine, chairman of Chase Econometrics.

"There is a 12-month period between a move in the dollar and a move in the trade deficit," added David Wyss, senior vice president and financial economist for Data Resources Inc., a forecasting firm.

He called September's deficit of $15.5 billion "a glitch." That analysis was borne out by the Commerce Department, which said the September totals were inflated because of the slow processing by the Customs Service of trade data for July and August. In adjustments to correct the lag in reporting, the Commerce Department lowered the September total to $12.8 billion -- more in line with the monthly averages -- and raised the unusually low figures that first had been reported for July and August.

The totals for those three months, however, portrayed the same bleak picture of high trade deficits.

Commerce Department figures showed that October's export total of $17.4 billion was $500 million less than the monthly average from January through September. In another bad sign, exports for the first 10 months of the year, which totaled $178.2 billion, registered a 1.02 percent decline from the same period last year.

"Until we see better orders for U.S. exports," said Data Resources' Chimerine, "it's hard to see how you can get a major turnaround in the trade picture. At this point, our feeling is that the trade deficit is a serious problem. There's been no fundamental improvement."

October imports totaled $28.8 billion, a slight decline from the monthly average so far this year. This came despite an increase in oil imports as dealers began building stockpiles for the winter.

But imports of foreign cars dropped sharply. The value of Japanese cars, for instance, fell a surprising 44.2 percent from September, to $1.1 billion, but the September figure was unusually high.

Most surprising, steel imports dropped a sharp 48.7 percent from last October's totals, most likely as a result of President Reagan's year-old program to cut the amount of foreign steel sold in this country. Import penetration declined to about 17.3 percent of the U.S. steel market -- the first time it had fallen under Reagan's target of 18.5 percent. Until now, foreign steel had been taking about one-fourth of the domestic market.

The biggest drop in steel shipments -- 60.6 percent -- came from the European Community, which currently is engaged in a dispute with the United States over future import limits.

The United States continued to maintain large trade deficits with most major countries. Once again, the deficit with Japan led the list, although the $3.2 billion gap for October marked a sharp decline from September's $5.1 billion.

Carried through for the full year, the U.S. trade deficit with Japan will total about $48 billion -- larger than America's entire trade deficit in 1982.

Other U.S. deficits in October were $2 billion with Canada, $1.5 billion with the European Community, $1.2 billion with OPEC nations, $966 million with Taiwan, $739 million with West Germany, $563 million with Britain, $449 million with Korea and $442 million with Mexico.