Thin film polarizer reflects about 4%~5% p-polarized light,when the laser beam is incident on the thin film polarizer at Brewster′s angle,which makes the laser beam reflected by intra-cavity thin film polarizer contain depolarized light (s-polarized light)and unignorable p-polarized light.This will result in intra-cavity depolarization rate measurement inaccurate.Intra-cavity depolarization rate measuring inaccuracy caused by intra-cavity thin film polarizer reflecting p-polarized light in an outputting p-polarized Nd:YAG resonator is experimentally investigated.The results show that,for different output mirror transmission in a outputting p-polarized light Nd:YAG resonator,there is obvious difference between the measured depolarization rate plots,when the p-polarized light caused by reflecting effect of thin film polarizer is not filtered from the depolarized light.And that tends to be consistent,if the p-polarized light are filtered from light reflected by the thin film polarizer.The investigation also shows that,for a outputting s-polarized Nd:YAG,the the depolarization rate can be directly measured,because the depolarization light,which is s-polarized light,contains little p-polarized light.