A quarter wave plate (QWP) is frequently used in a pulse Nd : YAG laser resonator and its function is determined by its location and the direction of the fast axis in the resonator. This paper gives a detailed analysis of the application of QWP in a typical pulse Nd : YAG laser resonator. Particularly, a method to control the polarization of the laser beam by adjusting the fast axis direction of QWP is proposed.