In order to test mild aspheric surface directly without other null optics, the digital plane method is proposed. When departure of the tested aspheric surface is mild, a sphere mirror can be used as the reference surface. The phase distribution can be measured swiftly by the digital interferometer. The surface error can be -obtained by subtracting the theory wavefront error (the value of the digital plane) from the phase data and eliminating the translation errors through least-squares fitting. The basic principle and theory of this method are analyzed and researched, and the testing model and flow chart are established. Meanwhile the experiment is carried out with a mild aspheric mirror by this method, the PV and RMS of the surface error are 0.173l and 0.018l (l is 632.8 nm), respectively. We also design and make a null corrector to the asphere for contrast, the differences in PV and RMS error between them are 0.026l and 0.001l, respectively.