Larry Hanft got an unpleasant surprise last week when he showed up at Iowa State University to pay his tuition and fees. A $1,400 Pell grant he was expecting won't be coming through after all.

The reason: Hanft, 32, is getting benefits under the GI Bill while he attends college.

Hanft is one of thousands of Vietnam veterans returning to college this fall to find they have lost federal financial grants -- aid they were counting on to help pay for their education -- because of a provision in last December's continuing budget resolution that changed the way their GI Bill benefits are counted in gauging eligibility for educational grants sponsored by Sen. Claiborne Pell (D-R.I.).

Veterans groups call the action another slap in the face to those who fought in the nation's most unpopular war.

Some members of Congress agree, and bills have been introduced to repeal the provision.

But even if they were passed today, the bills would have no effect on this school year.

About 89,000 veterans had Pell grants for the 1981-82 academic year, averaging $840 each. This year, more than 25,000 veterans may lose Pell grant aid totaling more than $25 million, Department of Education officials estimate. Congressional Budget Office figures show the loss may be double that.

The new law changes the treatment of veterans benefits in determining eligibility for Pell grants for needy students. In prior years, GI benefits were counted as part of a family's income, which was considered in an overall needs assessment. A low income meant a high Pell grant.

A veteran with a wife and child gets about $400 a month, about $3,600 during a nine-month school year. Now the money is being counted as straight financial aid, regardless of need, and many poor veterans, especially at less expensive schools, have lost their eligibility.

Some of the affected veterans also are angry because they did not know the aid cut was coming until they arrived on campus. Jerry Sullivan, director of financial aid at Iowa State University, said Friday that he did not get final regulations on the change from Washington until mid-August, a week before school opened, "so we had no way to notify students." Many of the 800 veterans on the 25,000-student campus are "still shell shocked" by the news, he said.

One of the irate veterans is Hanft, who was wounded as a Special Forces sergeant in Vietnam in the late 1960s.

He, his wife and their baby live in a mobile home. Both parents work part time to help with college expenses. But Hanft said the loss of the Pell grant means he will have to drop out of Iowa State, which costs about $5,000 a year for someone living off-campus.

He is considering enrolling in a nearby junior college. "I lost quite a bit of blood over there Vietnam ," he said. "I figure they owe me that aid ."

Sullivan called the change "an outrage" because the veterans who were eligible for Pell grants before still are needy. "These are guys who are older, probably with a wife and kids, who quit a job to take a shot at going back to school. Now all of a sudden I have to tell them the rules of the game were changed," he said.

At a House hearing called earlier this month to discuss the problem, Edward Elmendorf, deputy assistant secretary of education for student financial assistance, said the Reagan administration wants to make the change permanent.

"We appreciate the service of our military veterans," he said. But he added, "The most important issues seem to be those of equity and unnecessary duplication of federal expenditures. It does not seem appropriate or fair for some students to receive what is in effect a double federal payment when some of the funds could be going to more truly needy students."

Advocates of the veterans' position counter that GI benefits were intended to help a veteran readjust, and make up for time and experience lost to those who did not have to serve. "Now they are being penalized for receiving veterans' benefits," said an aide to Rep. Robert W. Edgar (D-Pa.), chairman of the House Veterans Affairs education subcommittee. "It's a double blow."

Only one witness at the hearing, besides Elmendorf, supported the change in the law. Miriam Rosenberg, national director of a private college student group, said that in days of budget cuts, poor students with no other financial aid should get Pell grants first. She noted that "it is with a sincere abhorrence of internecine competition" in the education community that she testified against the veterans.

The American Legion, Veterans of Foreign Wars and Vietnam Veterans of America all support the bills to repeal the change.

The VFW's James N. Magill said at the hearing, "We find it ironic that individuals who avoided military service or received other than honorable discharges . . . qualify for these grants when veterans are being denied. We are not asking that veterans receive special consideration, but that they be eligible to compete on an equal basis for the same benefits available to their nonveteran peers."

John F. Terzano of the Vietnam veterans' group said in a phone interview, "Our problem is that ever since Ronald Reagan came in we have had to fight for programs we had before: counseling, the Agent Orange work group and now Pell grants. They all are being cut and slashed.

"The president is good on the rhetoric, but the reality of a lot of his programs is decimating a lot of people around the country."