The Reagan administration announced yesterday that it is sending 100 Green Berets and the U.S. destroyer Thorn to Liberia as a demonstration of support for the military government of that West African nation.

The State Department, calling the decision "an expression of strengthening ties " between the two countries, said the U.S. Army Special Forces team would be in Liberia for 30 days to take part in training exercises.

State Department officials said Master Sgt. Samuel Doe, 29, who led noncommissioned officers and privates in a bloody coup that toppled the government of William L. Tolbert, a year ago this month, had asked for such a demonstration of U.S. military support.

The groundwork for the actions announced yesterday was laid in a meeting last month in Monrovia between Doe, who is chief of state, and Lannon Walker, acting assistant secretary of state for African affairs, according to sources.

Immediately after the military coup, which involved execution of Tolbert and 27 others, the Carter administration halted a Pentagon training mission and raised questions about continuation of the U.S. aid program.

Within seven weeks after the coup, however, the United States sent a high level delegation headed by the then-Assistant Secretary of State, Richard M. Moose, to improve relations with the new leadership. Moose made three return trips to Liberia before leaving office Jan. 20.

Washington has been concerned that Liberia, which was founded by expatriated American slaves and has had traditorally close American ties, might swing into alliance with Marxist Ethiopia or Muammar Qadaffi's libya. Both those countries made early bids for close relationships with the new Liberian leaders.

The dispatch of Green Berets and the naval vessel is particularly symbolic because of the timing. The Special Forces team is to arrive in Liberia on April 10 from Fort Bragg, N.C., and the destroyer is scheduled to visit Monrovia April 12-15.

April 12 is the anniversary of the military coup and execution of Tolbert, who was shot three times in the head by soldiers who broke into his mansion about 1 a.m.

State Department officials discounted a news report that Sgt. Doe is preparing to announce the ouster of Cuba military advisers at the same time as the arrival of the Americans. U.S. officials said they know of no Cuba advisers in Liberia.

In announcing the Liberia aid, State Department spokesman William Dyess said it is a response to the "desire of the Liberian government to increase its defense capabilities and a willingness on our part to assist them."

He said that the military coup in Liberia was based in part on discontent in the lower military ranks about working and living conditions and that U.S. aid is intended to improve such conditions.

In fiscal year 1981, the United States is providing Liberia with $1.7 million in foreign military sales credits and $449,000 in military training assistance.

Pentagon officials said that in addition to the Green Beret team, five other teams of U.S. military training personnel are scheduled to be in Liberia at various times this year.