The Treasury Department has begun formal meetings with congressional sponsors of tax simplification as it tries to come up with a plan acceptable to them and to President Reagan.

Treasury Secretary James A. Baker III and Deputy Secretary Richard G. Darman met in Baker's office last Thursday with Reps. Richard A. Gephardt (D-Mo.) and Jack Kemp (R-N.Y.). And last Monday the two chatted in Baker's Foxhall Road home with the principal senatorial sponsors, Bill Bradley (D-N.J.) and Robert W. Kasten Jr. (R-Wis.).

Before this round of meetings, the new team at Treasury had spent most of its time getting up to speed on taxes and other issues, and meeting with outside interest groups about tax simplification.

Baker has promised to send some kind of revision of the department's November tax-simplification proposal to Congress around May 1, with presidential endorsement. But he has given almost no details of its content and has not said whether it will be a specific bill or a set of recommendations.

So far, the various factions, although eager to work with one another, haven't engaged in hard bargaining. Gephardt said in a telephone interview that the Thursday meeting was "just a walk-through of the bills, where the tough problems are and getting the issues out on the table. It was not a real attempt to negotiate a bill."

Baker is attempting to keep the negotiations as quiet as possible. A Treasury spokesman would not confirm that the meetings had taken place or describe what was discussed.

Meanwhile, the House Ways and Means Committee is planning its second day of hearings on tax simplification next week. Next Tuesday, the congressional sponsors of tax restructuring -- possibly as many as 10 or 15 -- are to testify before the panel. Committee Chairman Dan Rostenkowski (D-Ill.) is also maintaining regular contact with Baker.

Senate Finance Committee Chairman Bob Packwood (R-Ore.) has met with Baker three times in the last month or so. But he plans to let the Ways and Means panel go first on consideration of a simplification bill, and probably will not hold committee hearings until the revised Treasury proposal has been sent to Capitol Hill.