The theory of thermal-induced-depolarization is analyzed. The distribution of depolarization is numerically simulated, and the result is verified by the experiment of the polarized light polarization interference. The experimental result is consistent with the numerical one, which shows that the distribution of depolarization in the cross section of Er:YAG rod is a crisscross and increases with the pump energy. The direction of the worst depolarization is along 45° angle with the polarization. A simple way is illustrated to measure the thermal lens focal length of the lamp-pumped Er:YAG laser with the critical stability condition of the cavity. The equivalent thermal focal length of Er:YAG crystal under different pump powers is also measured.