A time-dependent, two-dimensional theory is developed to calculate thermally induced stress, strain and deformations by use of separation of variables in Fourier law and thermoelastic potential method in stess-strain equilibrium equation. The beam distortion induced by thermal effects for calcium fluoride, potassium chloride and fused silica windows at 1.315 μm are deduced and analyzed. The calculations show that thermal lensing affects on beam the most among those three factors and birefringence can be neglected. Moreover, calcium fluoride window has the smallest PV value in beam phase distortions of them and potassium chloride window results in beam distortion the most badly.