Abstract: Based on the traditional Foucault tester, a quantitative evaluation method that improves the tester by digitized technology is proposed. Under the control of PC,knifeedge cuts the image point along XY plane by a fixed step size. A series of shadow patterns are captured through CCD in real time. Proceeding from the theory of geometric optics,the shadow patterns captured from Xaxis slices and Yaxis slices are analyzed and processed, the slope information of optical element surface is obtained, and the RMS and PV values of wavefront aberration for the optical element under test are calculated. In this way,the quantitative evaluation to the optical element surface is performed. The system configuration, principle and measurement process are described in the paper.