Flash floods in small- and medium-sized rivers mostly located in ungauged or data-shortage mountainous areas in China are featured by sudden happening, less concentration time, short forecasting lead-time and wide distribution. The primary purpose and task of flash flood forecasting for small- and medium-sized rivers is for early warning, therefore priority should be given to automatic forecasting, with an aim to realizing timely early warning to avoid loss of life and to reduce disaster losses. This paper analyzed the characteristics and difficulties of small- and medium-sized rivers flood forecasting, put forward the thinking of small- and medium-sized rivers flood forecasting as well as practical forecasting models and methods. A flash flood forecasting approach based on distributed hydrological model was proposed in this study and applied in flood forecasting and early warning in Tunxi river basin on the upstream of the Xin ’anjiang River, hoping to provide a research reference for the on-going national project of development of flash flood forecasting and early warning system. The research shows that the distributed hydrological modeling is an effective way to flood forecasting for small and medium-sized Rivers with shortage of data, and the flood forecasting technology based on distributed hydrological model can meet the practical requirements of automatic flood forecasting and early warning for small- and medium-sized rivers.