In order to simply obtain the phase of a surface in phase measuring profilometry (PMP), a fully automatic phase unwrapping method based on projected double spatial frequency fringes has been deduced. In this method, a series of formulas are deduced by analysing the relationship between the orders of and phase of the two fringes. Since the phase unwrapping is done on each point separately, the error propagation is avoided. Experiment has proved this method feasible, and a good measurement result is obtained.