The method of measuring new first grade gauge block by phase shift interferometry has been studied. Firstly, multi-frame interference patterns are captured by CCD. Then the shape of the measured surface of the gauge and the assistant reference flat to be wrung are obtained by phase shift arithmetic. In particular, the unwrapping wavefront principle and technology when there are step and discontinuity in the gauge interference patterns are studied, and the discrete wave error is calculated to fit to a continuous and smooth surface. Measurement of guage block length is discussed, and measured result about dynamic argument of gauge block working face length is presented.