Phase Measuring Profilometry (PMP) by a composite grating is a non-contract and real-time optical three-dimensional (3D) measurement method.By analyzing the composite grating,it is found that the measuring precision is related to the period of shift-phase gratings inside the composite grating when the frequencies of the spatial modulating gratings inside the composite grating are chosen properly.A period eigenvalue depending on the profile of the measured object is founded.It leads to a least Root-mean-square (RMS) measuring error.Therefore,the measuring precision can be improved by optimizing parameters of composite grating.When a set of designed composite grating patterns with different period of shift-phase grating are projected orderly by digital light projector,the corresponding deformed composite grating patterns are detected respectively to retrieve measured object with different RMS measuring error .The optimized composite grating can be obtained by RMS measuring error.Both the simulating and actual experimental results prove that the designed optimized composite grating can improve measuring precision.