For a large space building, the favourable fields of temperature and velocity are the premise for a comfortable thermal environment. The key to the settlement of approprite temperature and velocity fields is to find the way of forecasting surface temperature. In the paper, a heat equalization equation for coupled thermal conduction, convection and radiation in a large space building is founded. A theoretical method, by use of Gebhart absorption coefficient and a simple model for vertical temperature distribution, is put forward for calculating the surface temperature of the building. The calculation results are in good agreement with that of scale experiments presented in a certain overseas reference. It shows that it is an effective method for predicting the surface temperature of a large space building.