It is only the relative phase distribution between the tested object and reference plane that is attained after phase demodulation and phase unwrapping in profilometry. However, the objective of 3D shape measurement is to obtain the object′s height distribution .The nonlinear mapping relationship of both phase and height is analysed and deduced in a general experimental setup. The calibration scheme is designed. When the reference plane is shifted, its corresponding phases have been measured. According to the shifted distance and the phase increment ,the mapping relationship between phase difference and depth of the object can determined by the least square interactive method. Finally, a practical experimental system is calibrated and compared with Hung′s method. The experimental results and theoretical analysis show that the new method is characterized by the high accuracy, easy to handle ,lower the complexity of calibration .