The principle of the energy equilibrium is applied to establish the relationship between heat loss and supply， indoor and outdoor temperature， supply and return water temperature of heating system for a building. A numerical simulation model of supply-return water temperature is deduced. The heating system mathmatical model for the office building in Beijing area is developed Based on the hourly air temperatures at the automatic weather observation station and observed indoor temperature from the office building in February 2010 in Beijing area，a simulation experiment of hourly prediction for next 24-hours is executed. The test results show that the simulated and observed supply and return water temperature changed significantly when the outdoor air temperature changes， it is inversely proportional relationship between them， the correlation coefficient is -0.95， the simulated indoor temperature has remained near the designed temperature. There is a consistent trend between simulated and observed heating， as outdoor air temperatures rise， the simulated heating declined， it can save nearly 8 % for heating. This means the simulated model of supply-return water temperature has a good quality.