Secretary of State Alexander Haig is attempting to set up a meeting with the foreign ministers of Mexico, Canada and possibly Venezuela within the next few weeks to put some substance into the Reagan administration's faltering proposal for a "mini-Marshall plan" in the Caribbean and Central America, according to diplomatic sources here.

Although the Caribbean Basin Plan has come under increasing criticism, one diplomat here said, "It would be wrong to say the plan is falling apart." Conservatives in Washington are worried about the cost of the plan, and spokesmen for the Mexican and Canadian governments fear it may be a simplistic attempt to isolate Cuba and left-leaning governments such as Nicaragua and Grenada.

"There was never anything so structured that it could fall apart," the diplomat said. "This meeting would be one way that the Reagan administration could show it does have some cohesiveness to its foreign policy."

No date has yet been fixed for the meeting, but several possibilities are being discussed for the first two weeks in July, probably in the Bahamas. Both Canada and Mexico have agreed to attend if a suitable time can be found, according to diplomatic sources.

The meeting between Mexican Foreign Relations Minister Jorge Castaneda and Haig would be the first concrete test of the "good feeling" established by Reagan and Mexican President Jose Lopez Portillo at their Washington summit earlier this month.

It would also allow Washington to give at least some appearance of momentum and possibly some shape to a key element of its regional policy before a meeting of foreign ministers from more than 20 countries scheduled to take place here Aug. 1 and 2 in preparation for October's North-South summit conference.

Other meetings currently being promoted by various governments in the region and likely to at least touch on the aid proposal include a gathering of Central American foreign ministers, perhaps also in July, and a proposed summit of heads of state, including Central American countries, Mexico, Panama and perhaps Venezuela, although not the United States.

Reagan officials have indicated that the administration would contribute a large portion of the money to a regional aid plan, with other countries such as Canada, Mexico and Venezuela contributing lesser amounts.

Diplomatic sources say the Reagan administration is especially interested in stimulating private investment in the region, offering seed capital and, perhaps, some sort of insurance incentive to potential investors.

Some administration officials have suggested that the plan may consist of a collection of bilateral programs, which could allow Mexico or other nations to aid the area's leftist governments while Washington dealt with other countries it considers more sympathetic and more likely to create the atmosphere for private investment it seeks.

In such a context, Mexico particularly has expressed grave reservations about the direction the program could take, especially given the Reagan administration's obsession, from the Mexican viewpoint, with East-West conflict.

Even before Reagan took office, there was talk in multinational lending organizations of a massive international aid effort in the Caribbean, but it was conceived as a program designed by the various countries of the region to meet their specific needs, with the United States as only one of several potential donors.

In a much heralded first step toward such a goal, Mexico and Venezuela -- the area's two major oil producers -- signed an agreement last year to sell petroleum to what are now 11 of the basin's poorest countries on concessional terms that allow up to 30 percent of the cost to be used for local development programs. But more extensive efforts have failed to materialize.

For its part, Canada already is involved in an extensive program to the Caribbean and this week announced plans to double its total development assistance to the Commonwealth countries of the region during the next five years.

Regional concerns that the Reagan administration would use the program as an economic weapon in the growing East-West confrontation were heightened in recent weeks when the United States tried to exclude Grenada from a $4 million grant to be disbursed in the region by the Caribbean Development Bank. Regional governments last week said they would not accept any of the money on those terms.

Mexico, meanwhile, has demanded guarantees that the plan not contain any military element, that "its purpose would be to help the population of the region and it would not be a political instrument designed for the fight against the Soviet Union or communism in the region," and that no country in the region ought to be automatically excluded from its benefits, according to the Foreign Ministry.

After the meetings between the Mexican and U.S. presidents, Castaneda made it clear that while Reagan had accepted these conditions in principal, Mexico had not agreed to accept or participate in the plan, but simply to explore and discuss its possibilities. Castaneda also suggested that the key stumbling block would be the question of which countries finally would be included in any such initiative.

Mexico has had a long and friendly relationship with Cuba and repeatedly has expressed its support for Nicaragua's leftist government, both of which have been the objects of increased U.S. hostility under the Reagan administration.

"This is a difficult affair," said Castaneda, "and an affair that requires quite a bit more consultation among its promoters."