The U.S. merchandise trade deficit declined in April for the second month in a row, decreasing to $13.3 billion, the government reported yesterday. The Reagan administration predicted further improvements in the trade picture will spur economic growth and create new jobs.

The April deficit was $312 million less that the previous month's and lower than the monthly average of $13.6 billion for the first four months of 1986.

On the basis of the trade figures so far this year, the United States appears headed for the first improvement in its annual trade flow since 1980, when the deficit stood at $36.4 billion. The shortfall has steadily worsened through President Reagan's tenure, setting new yearly records until it topped off near $170 billion last year.

At the present rate of improvement, this year's trade deficit should drop to $163 billion, but administration officials predict that further improvements will cut the deficit to about $150 billion.

"The worst is over. Probably a slow turnaround is in progress," said Allen Sinai, chief economist for Shearson Lehman Bros. of New York.

Commerce Secretary Malcolm Baldrige echoed that view with an unusually upbeat statement on the trade deficit. He said the trade gap has declined even more than the monthly totals show when inflation and seasonal variations are taken into account.

With the 35 percent fall in the value of the dollar against other currencies over the past 20 months, imported goods have become more expensive in the United States and U.S. exports less costly in overseas markets. The lower dollar helps the sale of U.S. manufactured goods and farm products at home and overseas, but it also makes the trade deficit appear worse than it really is, economists said.

"We will see occasional monthly setbacks, but the improving trend will continue," Baldrige predicted. "The success of domestic producers in regaining market share will have a sizable effect on growth in output and employment this year."

Nonetheless, Jerry Jasinowski, chief economist of the National Association of Manufacturers, said the U.S. trade deficit will continue high as a result of the failure of Western leaders to agree to stimulate world growth during this week's economic summit in Venice.

"These poor numbers illustrate how the Venice meeting missed the mark by failing to support more stimulative world growth," said Jasinowski.

"More stimulative world growth action is what is necessary to provide markets for U.S. exports. Our exports are not doing better because nobody's buying anything in other countries. World growth is just too slow.

Speaking of the leaders of the United States, Japan, West Germany, Britain, France, Canada and Italy who met in Venice, Jasinowski said, "These guys had a great chance to strike a positive chord in stimulating world growth, and they failed to do so."

The Reagan administration has been pressing Japan and Germany with little success over the past two years to provide faster growth in their economies. Without that, Jasinowski said, "we are just going to get dribs and drabs of improvement" in the U.S. trade deficit.

Senate Democrats, about to begin consideration of legislation aimed at attacking unfair trade practices and sharply lowering the deficit, said the April decline is too small to offer substantial help to the economy.

Sen. Don Riegle Jr. (D-Mich.), a constant critic of administration trade policies, called the figures "devastating." Senate Finance Committee Chairman Lloyd Bentsen (D-Tex.) said, "Our trade situation continues to muddle along, piling on more international debt month after month, and it will continue to do so until we put an effective trade policy into place."

"We are only fooling ourselves if we think we'll ever see a sustained, healthy improvement in U.S. exports without doing something about foreign trade barriers," said Bentsen.

But the head of another major business organization, William Lilley III of the American Business Conference, urged lawmakers to "avoid measures that will give our trading partners an excuse to enact their own protectionist laws, thereby impeding the progress we are making in expanding our exports."

"Restrictive trade laws would curtail rather than encourage the decline in our trade imbalance," Lilley added.

April imports amounted to approximately $33.5 billion, a $1.2 billion decline from March, but still $1.1 billion greater than the average for the first three months of the year.

Exports totaled about $20.1 billion, $1 billion less than the March total but $1.4 billion higher than the January-February-March average. Exports of both manufactured goods and farm products dipped in April, the figures showed, with overseas sales of manufactured products down $800 million.

On the import side, the April decline reflected decreases in U.S. sales of foreign-made steel, data-processing and office equipment and power generators. Imports of clothing, shoes and telecommunication equipment remained at their March levels.

Sinai, the Shearson Lehman economist, called the drop in manufactured imports "a healthy sign" that suggests "that maybe Americans are beginning to substitute domestic for foreign goods." But he cautioned that the import drop may have been caused by the country's sluggish economy.

Once again, the United States ran its largest deficit with Japan, $4.9 billion, virtually the same as in March.