Abstract: To examine the change of runoff and its reason is an important scientific issue in forest hydrology. In this study, we performed the trend and inflection analysis on the time series of daily precipitation and runoff in the Pengchongjian small watershed from 1983 to 2014 by Mann-Kendall test method, a site with abundant precipitation . Using the empirical statistics method, we analyzed the effects of precipitation variation and vegetation restoration on the runoff and its contribution rates. Furthermore, we calculated the critical value of the hydrological effect of vegetation restoration on the annual precipitation. The results showed that the year 2003 was a consistent abrupt point for annual precipitation and runoff. Compared to the baseline period (1983-2003), annual precipitation and the depth of runoff in the changing period (2004-2014) decreased by 8.7% and 29.2%, with the averaged annual decrease of 12.7 and 22.1 mm, respectively. The averaged depth of runoff in spring, summer, autumn, winter, and in the whole year decreased by 100.2, 105.8, 25.2, 23.4, and 243.0 mm, respectively. The contribution rates of the depth of runoff to precipitation varia-tion were 58.9%, 71.6%, 65.5%, 35.0%, and 57.1%, respectively, while the contribution rates of vegetation restoration were 41.1%, 28.4%, 34.5%, 65.0%, and 42.9%, respectively. The hydrological effect of vegetation restoration was attributed to the annual precipitation, with a critical value of 1181 mm. Vegetation restoration increased annual depth of runoff when annual precipitation was less than the critical value, and increased that when annual precipitation was higher than the critical value. Therefore, the critical value might help to explain the difference in contribution rates of vegetation restoration to runoff in different watersheds and serve as one of the important reasons for the debate and divergence of forest restoration impacts on runoff.