The paper briefly reviews the research work on nonlinear relationships between rainfall and runoff for a watershed at home and abroad. According to experimental data of overland flow on field in northwest regions of China and those of channel and laboratory watershed in the laboratory,there are H groups that consist of 316 events on field and 2 groups that consist of 234 events in laboratary. Nonlinear relationships are further proved and the relationships between the index of nonlinearity and characteristics of watershed are preliminarily analysed, the mathematical formula of which is as follows.