Under thermal load in a fire accident, there are many factors leading to the breakage of window glass. To identify the influence of these factors, a closed chamber experimental device was used. A series of orthogonal tests with 7 factors and 2 levels were designed. It was found that the glass thickness, the glass edging and the rate of rise of temperature from heat source are the most important factors to first crack. And then is the glass size, the width of the unexposed glass surface. On the other hand, the thickness of the filler (gypsum powder) in the frame and the distance to the heat source are less important. The median of the distribution of temperature difference and thermal stress got from these experiments is 129.2℃ and 69.13MPa, respectively.