In this paper we discuss the polarization characteristics of a He-Ne laser with a rotatable quartz plate in its cavity (LRQP). We have found that the output of the LRQP is two mutually Perpendicularly plane polarized light. Most importantly, it is obsevered that the polarization orientations of the front beam and the back beam from laser tube are different. There is a relative rotatory angle between them. When we rotate the quartz,the polarization orientations of both the front beam and the back beam vary with the rotation of the quartz plate. And the relative rotatory angle between the front beam and the back beam changes too.