The administration issued a blast at tuition tax credit legislation yesterday, charging it would change government policy to provide more financial aid to those attending private schools than to public school students.

In a letter to Ways and Means Committee Chairman Al Ullman, (D-Ore.) Treasury Secretary W. Michael Blumenthal and HEW Secretary Joseph A. Califano Jr. said the net effect of tax credits would be "a major and undesirable change in federal policy." They said the credits would provide three or four times more federal support for private school children than for public school children.

They also said the credits would be inflationary and unconstitutional.

Ullman has scheduled hearings for today and tomorrow on legislation that would allow students or their parents in college, vocational schools and in some cases parochial and other private schools to subtract $250 to $500 from their taxes each year for tuition.

A tax credit is taken directly from taxes owed as opposed to a deduction or exemption which is subtracted from income before taxes are calculated.

Califano and Blumenthal said families with incomes over $30,000 would get 20 percent or more of the benefits under the two leading tax credit bills. The bills would cost $1.2 billion and $4.4 billion, respectively, during the first year.

The administration's own bill supports increasing the amount of money available for grants for needy families.