A quantitative analysis of the 2nddegree burn injury of human tissues and the relevant mathematical model are presented in this paper. Unlike the general criterion extensively adopted by doctors that pay much attention to the degrees of the skin burn damage such as the area, depth and color of the damaged skins, the paper emphasizes the cause-and-effect of the 2nddegree burn injury of the human tissues. Based on the theoretical study and the experimental observations, the paper presents a quantitative analysis method to establish a relationship between the 2nd degree burn and the physiological parameters of the human body as well as the physical parameters of the exposing surroundings. The comparisons of the theoretical study with the experimental data have been made and the results are quite satisfactory. The application of the mathematical model in the test of thermal protective performance of fire protection clothing is also presented.