The retrace error is one of the most important factors for high precision testing of Fizeau interferometer. The wavefront of interferometer is the mainly factor of retrace error. So in order to measure the wavefront of interferometer, a method of testing the wavefront of interferometer is proposed, which is based on the measuring function of interferometer itself and is very easy to carry on. Just an additional reflection flat is needed. In this method at least three cat,s-eye test results are used to get shearing of the wavefront, which is used to solve the wavefront map by shift-rotation absolute testing algorithm. Experimental results show that the wavefront testing uncertainty is about [root mean square (RMS)] 15 nm without the Power term. It can be utilized to test the wavefront quality of interferometer precisely, and the retrace error farther.