President Reagan, acting on his pledge to penalize Poland's military rulers for banning the Solidarity trade union, yesterday was preparing to suspend Poland's favored trade status with the United States.

State Department spokesman Alan Romberg said Reagan plans to sign, very soon, a proclamation suspending Poland's most-favored-nation status.

Romberg said the action "was seen by the president as a tangible way of expressing our deep displeasure over the Polish government's action to abolish Solidarity on Oct. 8."

On the day after that crackdown, Reagan announced in a radio address his intention to retaliate against what he called the "outrage" committed by the Warsaw regime. His formal action was delayed while the administration studied the legal mechanisms involved in withdrawing the trading status, which is granted by Congress on the president's recommendation.

Romberg said that the president, after consulting with congressional leaders, decided to act under provisions of the 1974 Trade Act, which gives him authority to terminate trade agreements under certain conditions.

The spokesman said Reagan will decree the suspension on the basis of an administration finding that Poland has failed since 1978 to fulfill its obligations as a member of the General Agreement on Tariffs and Trade (GATT). Specifically, Romberg said, Poland has not met its commitment to increase the total value of its imports from other GATT members by 7 percent a year.

"On these grounds, the United States would have been fully justified in taking action such as suspension of Poland's MFN status years ago," Romberg said. "We decided against such action on the basis of Poland's then relatively favorable record on internal liberalization . . . .

"This favorable situation in Poland was reversed by the official outlawing of the independent trade union Solidarity, leaving us without any reason to continue withholding action on our trade complaints against Poland."

As a general rule, MFN status guarantees that no nation will be charged higher tariffs than any other country, although there are exceptions.

In practice, all countries have the status except the Soviet Union and certain other Communist bloc countries.

U.S. officials estimate that Reagan's action might increase tariffs on about $100 million of the $200 million in Polish goods imported annually by this country. That theoretically could reduce the dollar volume of Polish sales to the United States.

The major effect would be on Polish textiles, with agricultural products, including such prime Polish exports as canned hams, remaining unaffected by the tariff hikes.

The administration's freedom of action was hampered by several factors. High-technology exports to Poland already are banned.

Limiting agricultural exports to Poland, which has severe food shortages, would increase hardship on the Polish people, and calling loans to Poland into default would harm western bankers.

A group of Polish-American leaders who met with Reagan yesterday said that they approved of his action.

However, one member of the delegation, Aloysius A. Mazewski, president of the Polish American Congress and Polish National Alliance, expressed disappointment that Reagan plans to continue U.S. grain sales to the Soviet Union, which has great influence over the actions of the Warsaw regime.

"We told him we were disappointed with that," Mazewski said in response to reporters' questions.

However, he added that he and his colleagues were aware that the hard currency the Soviets will have to pay for grain will detract from the reserves Moscow has available for military spending.