The runoff data was collected from Sunshuiguan hydrological station located in Sunshuihe river basin, Liangshan Prefecture, Sichuan Province, China. The runoff distribution and its tendency as well as the effects of precipitation and human activities on runoff since the last six decades were analyzed by application of the methods of Mann-Kendall trend method, cumulative departure curve inspection method, and double mass curve method. The results showed that:(1) The annual average flow had an obvious but insignificant decrease with a rate of 0.116 3 m³/s per year and a mutational point was observed in 1997. (2) The distribution of runoff was relatively uneven in each year, and the runoff was mainly contributed by the summer and autumn. The runoff significantly decreased in the spring and winter with a rate of 0.028 8, 0.070 9 m³/s per year, while the decrease was not significant in the summer and autumn. The flow mutation occurred in the summer of 1995. (3) The decrease of flow in the Sunshuihe river basin was mainly caused by precipitation and human activities, and the effects of the two factors on the flow varied in different periods. The decrease after 1975 was largely caused by human activities, with the highest impact rate reaching 89.16%. The research results could provide a theoretical basis for the runoff rule study, flood control and drought prevention, and water resources utilization in the mountainous areas of southwest China.