A new method of actively modifying the periods of projected grating in divergent lighting system is proposed. In the on-line three-dimensional (3D) inspection with phase measuring profilometry (PMP) based on Stoilov algorithm, the divergent lighting system results in the non-constant period of the fringe pattern on the reference plane, which will lower the measurement accuracy. Therefore, a novel method about modifying the periods of the projected grating is proposed, which improves the flexibility and accuracy of the on-line 3D inspection and avoids the complicated calculation and measurement of the system parameter. Through analyzing the wrapped phase distribution of reference plane with static PMP, modifying the periods of the projected grating gradually by iterative algorithm, the constant period of fringe pattern on the reference plane is achieved. Experiment proves the feasibility of this method.