Radial shearing interferometry has been widely used to test distorted wavefront. However, the result of wavefront measurement by using this technique includes two parts, one is the real phase information of wavefront under test and the other is the system errors in radial shearing interferometer. A calibration method of a radial shearing interferometer is presented. First of all, several fringe patterns of the phase difference distribution are obtained by a tested component with different positions. Then, the maximum likelihood method is used to combine the phase difference data and create a maximum likelihood function. After that, the wavefront under test and system errors can be separated. The simulation results show that the maximum likelihood method can deal with the issue of the calibration of a radial shearing interferometer. Furthermore, the method is used to reduce the effect of the system errors on extracting and reconstructing the wavefront under test.