The thermal depolarization in ring laser diode array side-pumped rod laser is studied. Based on the intensity distribution of laser diode (LD), a inhomogeneous heat distribution in the laser rod has been obtained by using the ray tracing method. And then, the transient temperature rise and thermal stress distribution are obtained by using the finite element method. In this paper, the relative thermal optic pass difference (OPD) and output beam patterns with depolarization are simulated at both thermal steady state and transient state. Simulation results show that the thermal induced depolarization grows with the input power, and the transient depolarization grows with the temperature rise and reaches to a steady state as well.