This paper describes the fundamental and performance of a self-developed phase-shifting digital interferometer. This instrument can be used to test wavefront distortion of laser rods whose aperture is 30 mm with accuracy better than λ/20, the repeatability better than λ/50. The instrument has four characters: magnification changing, focusing , luminous flux controlling, and changeover of adjustment and test. Software based on the platform of windows98 can finish interferogram sampling, processing and wavefront drawing, the operation is easy and convenient. The whole testing only spends 26 seconds. This instrument can also measure the refractive-index homogeneity, the flatness, the angle error, the wedge angle of crystal element. By testing the same samples with Zygo GPI-XP, both results were same.