A thermo-elasto-opticaldistortion model for the main amplifier of SG-Ⅲ prototype is presented in this paper, which consists ofthree modules, i.e., modules of heat transfer, thermo-elastics and optics. With thismodel, the residual thermal distortion of the main amplifier of SG-Ⅲprototype is investigated. The calculatingresults indicate that, with an optimization thermal recovery, the thermal gradient and theaverage temperature of the slab can be reduced to an acceptable value. The residualthermo-optical distortions can meet the design requirements. Additionally, a quantitativerelation between the thermal distortion and the linear thermal expansion coefficient ofthe laser gain medium have been also obtained.