In this paper, a non-null interferometric measurement method of X-ray mirror is proposed, which can achieve high precision interferometric measurement of zero ratrace error without splicing. A high precision plane mirror is used to calibrate the ratrace error of the interference system in the full field of view. By dividing the aspherical mirror into several sub-apertures, each sub-aperture can be approximately regarded as a plane, so that the ratrace error corresponding to the sub-aperture can be found from the calibration database, and the ratrace error of the entire aspherical mirror can be obtained by simple matrix splicing. Taking the X-ray elliptic cylindrical mirror as an example, the ratrace error of the non-null interferometric surface shape of the X-ray elliptic cylindrical mirror is effectively calibrated. Compared with the splicing interferometric method, the results of the two methods are consistent, which confirms the correctness of the proposed method.