Traffic flow forecasting provides important information for both traffic control and traffic guidance. It should be both quick and accurate. A short-term traffic flow forecasting method is given based on the macroscopic urban traffic network model. The model is established to describe the substantial mechanism of traffic flow movement and the topology of the entire urban traffic network. It can simulate the traffic movement in the urban traffic network, and forecast the traffic flow states in the near future accurately. Also, the method has a good real-time feature due to the macroscopic model. In the simulation experiment, microscopic model CORSIM is used as the practical traffic system, and the proposed method is used to forecast the traffic flow of it. The simulation results show that the method has a good forecasting effectiveness.