The axial spacing calibration between the two big compensators, the deformable mirror (DM) and the partial null optics in the adaptive null compensator (ANC), is investigated. A multi-null constraint method is proposed, in which the deformation of the DM is used to acquire the null measurements at different positions with the help of a calibrating mirror. Thus the multiple measurement equations are constructed to constrain the error coupling and the self-calibration of spacing distance in ANC is realized. The high efficiency of this method is verified by simulations and experiments. The direct measurement way is discarded in this method, which makes the integration between ANC and the whole interferometer possible.