Abstract: This paper concerns the thermal lensing effect that occurs as laser going through an optical window, and its influence on beam quality. By the method of impetus theorem, 2D solution of the thermal conduction equation is derived, in the form of generalized Fourier series. Then, the finite element method is applied to simulate the thermal deformation as a plane stress problem. Combined with the thermal deformation and the shift of the refractive index with the temperature, the wavefront distortion is calculated without considering the opticstress effect. Furthermore, the beam quality on far field is discussed. Temperature rise, thermal deformation, wavefront distortion and beam quality of fused quartz(SiO2) window and white bijou(Al2O3) window irradiated by the annulus beam are simulated a