Thermal effect is still the most serious problem: it restricts the high power and high beam quality of solid laser to be further enhanced. The efficient thermal management is an important approach to suppress the thermal effect. In this paper, the thermal effect in a gas-cooled laser diode pumped multislab Nd:glass amplifier operating at a repetition rate is investigated in detail both theoretically and experimentally. The three-dimensional distributions of temperature, stress, strain, and birefringence are calculated by a finite element analysis. Based on these data, the thermally induced wavefront distortions and depolarization losses are determined with considering six slabs and one laser head. It is revealed that the theoretical data are in good agreement with the experimental results: the total wavefront distortion is 6.77λ and a depolarization loss of more than 90% accumulates over six slabs when the heat deposition is 0.7 W/cm3.