An approximate solution for the determination of surface temperature by using integral method is presented in the paper. In contrast to many other methods proposed in the literatures for solving this problem, the method presented in the paper is rather simple and convenient for applying, and can also be used to the case that thermal conductivity is temperature-dependent. By comparison with direct solution of the problem, the formula pocesses high accuracy, and may be applied to the determination of the surface temperature, such as the insulated furnace inner wall temperature.