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 are studied when there are step and discontinuity in the gauge interference patterns. The discrete wave error is calculated, which is fitted to be a continuous and smooth surface. The measurement results of guage block length and dynamic argument are given.