Abstract: Based on the thermal elastic model with thermal-mechanical uncoupling approximation and commonly-used assumptions, the scaling law in thermal deformation of high reflecting mirror under laser irradiation is obtained by equation analysis. For the same model, displacement, temperature rise, stress and strain change linearly with laser power density. The numerical results are consistent with the theory. The conclusions lay the foundation for using scaled small model to investigate the thermal deformation of large-size mirror under laser irradiation, and provide specific guidelines for the model designing and transforming experiment results to the prototype.