Calling for diffraction limited imaging performance, extreme ultraviolet lithography (EUVL) demands unprecedented requirements for figure metrology. The asphererical surface of EUVL mirrors must be measured with higher accuracy. Root mean square (RMS) is 0.2~0.3 nm. The design of interferometric null metrology is focused on, which can produce such highly precise figure of aspherical surfaces. The null lens is splited into a transmission sphere and a single lens. The errors introduced by the interferometer and the transmission sphere can be calibrated, and the error from the single lens can be rigidly controlled to guarantee the system testing accuracy. The residual aberration of the null testing system is controlled to be smaller than 0.003λ in the system. The main error sources of the null testing system are analyzed, the parameters of environmental control are proposed as well as limite residual aberration of the transmission sphere. Furthermore, the error sources are classified and the measurement accuracy of our aspherical null testing system is evaluated, RMS reaches 0.245 nm. It can meet the requirements of EUVL optics testing.