Abstract: Based on the distribution of the residual stress in the window glass of manned spacecraft, the calculation model of stress intensity factor for thermal tempered glasses is formulated and reinforce performances are analyzed. By using the model, the effects of tempering degree and glass thickness, as well as the crack growths are discussed. It is pointed out that, at the same tempering degree, the fracture strength increases with the increasing of the thickness of tempered glass, but does not for untempered glass. The discussion of the stress intensity factor for tempered glass shows that it is unreasonable to take KIc+Kr as the critical stress intensity factor for tempered glass.