Based on summarizing the rule of rainstorm and snowmelt mixed flood, the structure of rain-on-snow runoff-generation is discussed; and critical temperature is used to determine the form of precipitation and snowmelt factor, taking into account rainfall volume of snowmelt. A rain-on-snow flood forecast model is developed by combining LL- I distributed hydrology model. The Kalangguer River, an internal river in Xinjiang Autonomous Region, is taken for example. It is indicated that the model has a higher precision of forecasting; its determinacy coefficient is greater than 0.80.