The polarization characteristic of a He Ne laser with an activity in its cavity (LAPC) is studied with laser physics and crystal optics. The experiments show that two modes of the laser are orthogonal linear polarized. There is an angle between the polarization orientation of the front beam and the of the back beam. Moreover, this angle is related to the angle θ, which is the angle between the optical axis and k. The experiment results can be explained by the analysis. The theoretic calculation matchs the experiment well.