Abstract: As the material of an optical window plane being transmitted through by high-power laser, some part of the laser energy would be absorbed and it may get thermal-deformation and become into thermal-lens. Here we begin from an analysis solution, which has got by us, of the distribution of heat-field in a circular material plane and consisted with the solid mechanics, then the expression of the thermal-focus length has been obtained. The effects different between materials of Al2O3 single crystal and fused silicon has been calculated and compared in 1.315 μm wavelength. Al2O3 single crystal plane is one of the optical material provided with high transparence and high threshold strength in wavelength 1.315 μm. But the ordinary and extraordinary light would be separated in the crystal. This e