The behavior of self-polarization emission in Nd:Y3Al5O12(YAG)/Cr4+:YAG lasers has been proved in some cases. However, the degree and direction of polarization were often sensitive and unstable. We experimentally observed different beam profiles versus the angle of the polarizer relative to the polarization direction of the laser. In order to explore the polarization mechanism, the dynamics of intracavity polarized eigenmodes was analyzed theoretically. Simulative results were well consistent with our experimental observations. It indicated that the linear self-polarization emission was a composite state rather than an intrinsic state. This study contributed to the improvement of the polarization stability in Nd:YAG/Cr4+:YAG passively Q-switched lasers.