The invention relates to a mountainous area reservoir area riverway water surface width estimation method. Further development is made on the basis of a river width model constructed under the naturalcondition, the relative reasonable and reliable mountainous area reservoir area riverway width estimation method is provided, the distributed model confluence calculation accuracy is improved, the problem about estimation of the small and medium-sized basin mountainous area reservoir area riverway width lake of observational data is solved, important essential data are provided for reservoir flood control regulation, hydraulic power generation, channel improvement and the like, the reservoir regulation scientificity is improved, and safety of downstream zones can be better and practically protected. The mountainous area reservoir area riverway water surface width estimation method is mainly applied to a drainage basin digital elevation model, data sources are stable and reliable, the computational efficiency is high, the result is objective and reasonable, automation generation of drainage basin riverway width and direct calling of a distributed hydrologic forecasting model are facilitated, and the development of digital hydrology, the distributed hydrologic forecasting model and reservoir scientific management is promoted.