The abrupt change of runoff series may occur in the changing environment. At present, researches on the contributions of runoff variation mainly focus on the annual scale, but pay insufficient attention to the seasonal scale. In this paper, the typical watershed of the Loess Plateau, Jinghe River Basin, was taken as the research object. Firstly, the wavelet analysis method was used to reveal the evolution of seasonal runoff series. Then the order clustering method and cumulative anomaly method were used to diagnose the variation of runoff sequence. Finally, the contributions of climate change and human activities to the seasonal runoff variation were quantified by making a comparison of slope change ratio of the fitted beeline between year and accumulative quantity. Results showed that:(1) The period of summer and winter runoff shift from small scale to large scale after the 1990s; (2) Runoff variation occur in spring, summer, autumn, and winter in Jinghe river basin are 1969, 1996, 1985, and 1990, respectively; (3) The contribution rates of human activities to runoff variation in spring, summer, autumn, and winter are 50.40%, 89.69%, 39.14%, and 84.59%, respectively. The contribution of human activities (mainly soil and water conservation measures) to runoff variation was dominant in summer and winter, which mainly affected the periodic shift of runoff after the 1990s.