For the purpose to overcome the difficulty of testing aspheric surfaces by null lens or CGH (Computer-Generated Hologram), the non-null testing was proposed. The basic principle and theory of testing asphere by digital plane, partial compensation and subaperture stitching interferometry(SSI) were analysed and researched, and each testing model and flow chart were established. Combining examples, an asphere of little departure with the aperture of 350 mm was tested by the digital plane and SSI, respectively. As results, the difference of PV and RMS error between SSI and partial compensation is 0.015λ and 0.002λ(λ is 632.8 nm), respectively. The prototype and setup for testing asphere by partial compensation were devised and developed, a precise convex asphere is measured by this method, the PV and RMS of the surface error is 0.183λ and 0.018λ, respectively.