A digital knife-edge testing system is established. The knife-edge testing system is used to quantificatively evaluate the wavefront of conicoid aspherical optical element. Root mean square (RMS) and peak-to-valley (PV) values of wavefront aberration of aspherical optical element are given. Measurement results of digital knife-edge testing system are compared with the measurement of interferometer. The measurement results show the consistency within 0.001 μm, which proves the feasibility of digital knife-edge testing system and the accuracy of the digital knife-edge testing system reported here. The measurement results are discussed in the end.