Thermal depolarization caused by birefringence is a major factor that limits the output power of linearly polarized Nd:YAG laser. Compared with conventional [111]- cut Nd:YAG rod, the thermal depolarization depends on the polarization direction for the [100]- cut Nd:YAG rod. The linearly polarized output power can be improved by suitable modification of the polarization direction of oscillate laser. The [111]- and [100]- cut Nd:YAG rods are used as the gain medium and the Cr4+:YAG crystal is used as the saturable absorber for passively Q-switched laser. The experiment demonstrates that in comparison with conventional [111]- cut Nd:YAG rod, highly stabled output with high extinction is achieved by using the [100]- cut Nd:YAG rod as the laser crystal. The extinction ratio is 5001 and polarized output stability is higher than 96.5%.