Abstract: In the forecast of dynamic systems, future values of system input varibles are often unknown, and influenced by those of system output varibles in the way of feedback. Such forecast problems often occur in social-economic systems. The forecast of freight quantity to be transported is such an example, which we met when we studied the problem of transportation in the Three Gorges on Yangtze River. In this paper,the cyclic forecast method is proposed to solve such problems.The general formof discrete time system model and the forecasting procedure are given.The forecasting error is analysed and the results are heuristic.