Vice President Bush says the Reagan administration would like to send the "undesirable" Cuban refugees back to Fidel Castro if there is a "humane and credible way of doing it."

Responding to questions from reporters here Saturday night on the first stop of a four-day political swing through Florida, Bush took a hard line on the Cuban refugee question that has agitated public opinion in this state and others where last year's wave of immigrants have settled.

Told by a reporter that Florida Gov. Bob Graham, a Democrat, is advocating "returning Cuban undesirables to Cuba," Bush said, "He's not alone. . . . Why should the United States have inflicted on it, not legitimate refugees fleeing Castro tyranny, but people who are pushed out of the jails and put on boats to come to this country? It's totally unreasonable."

Asked how the Reagan administration would react to another flotilla of small boats bringing refugees from Cuba to Florida, he said, "With an enormous lack of enthusiasm."

Bush said he was not certain what steps would be taken to prevent such an influx, but he declared, "We're not going to let Fidel Castro determine the immigration policy of the United States.