The study of human body's radiation heat loss and thermal comfort need to calculate the angle factor between the human body and the envelope surfaces. Firstly, the paper derived the mathematical equation to revise the result of numerical calculation of angle factor based on the discrete formula of the angle factor and the effective radiation area, and established the standard three-dimensional body model of Chinese human in sitting posture. Then, the paper calculated the angle factor between the human body and roof, wall, floor, respectively, and found that the angle factor between the human body and the front surface is greater than the rear surface. Compared numerical calculation of angle factor with semi-analytical calculation, it is found that there are good agreements between semi-analytical calculation and numerical calculation for the roof and vertical wall, but there is a larger error for the floor, and the paper has modified these errors.