Gansu Province is located at the northwestern China with a complicated weather condition and geography environment,the sparse runoff observation station is along the river ways,whereas above status,according to relational methods of conventional system hydrologic model and river network flow concentration theory a new flood forecast model called Auto-adapting Flood Forecast Model Considering with the Effect of Inflow into Reaches is established.In the model water storage variables and auto -adapting method of considering the effect of the inflow into reach are presented.So the lopsided relationship between input and output in the actual system is ameliorated and the problem of flood processing forecasting is solved by using the method.The results show that Auto-adapting Flood Forecast Model Considering with the Effect of Inflow into Reach es possesses a favorable application foreground on the arid and semi-arid areas in the northwestern China.