Abstract: A digital knife-edge test system is proposed based on the traditional Foucault tester. Quantitative evaluation for the wavefront of spherical optical elements was achieved by using this test system. The system configuration, principle and testing process are described in the paper. The surface of a spherical optical element with the radius of 1000mm and the aperture of 160mm was tested with the test system. The measured RMS and P-V values of the wavefront error of the optical element under test are given. The measured results are discussed.