Topmodel is a semi-distributed watershed hydrological model based on topography.It has been applied in many hydrological modeling and is a very successful watershed hydrological model.This paper introduced the results of predecessor application and the basic principle of the model,then took the inflow forecast of the Guxian reservoir for example to probe its application in rainfall runoff simulation in large catchments. The results show that the model has a strong applicability in this basin and can be used to predict inflow of Guxian reservoir in flood season,and can provide theoretical foundation for safe operation and optimal dispatching of the reservoir.