As the phase height model in traditional Phase Measuring Profilometry( PMP)has a larger storage space and higher calibration cost, this paper deduces an explicit function model of phase difference height according to the relationship between absolute phase and depth coordinate, and proposes a flexible calibration method based on unknown calibration plane. The model can calculate 15 model parameters only by 9 nonlinear pixel data, so it saves the memory space and reduces the computing cost. The proposed calibration method does not need any movement platform and position posture, and brings the advantage of small calibration board and well flexible ability. The simulation and experiment on the model and calibration method are performed. The simulation results verify the validity of explicit function model of phase difference height and the availability of the proposed calibration method. Moreover, an actual experiment for the model and the calibration method is done in a structure light system. The calibrated system is used to test a real objective, and the results show that the 3D curved surface has higher quality, which demonstrates the proposed method is feasibility.