Studying runoff change, and its causes is an important scientific issue in forest hydrology. Presently, most studies have quantitatively analyzed the contribution rate of runoff change and its influencing factors based on annual scales, whereas few studies have been conducted on seasonal scales. Therefore, the attribution analysis of runoff change on a seasonal scale needs further study. Based on the daily precipitation, evapotranspiration, and runoff from 1983-2014 in Pengchongjian, a small watershed, a Mann-Kendall test method was applied to analyze the abrupt point of annual precipitation and runoff. Ultimately, the slope change ratio of accumulative quantities (SCRAQ) method was used to separate the contribution rates of precipitation, evapotranspiration, and vegetation restoration between annual and seasonal runoff. The results indicated that:a consistent abrupt point of year 2003 was found both for annual precipitation and runoff; the results from the SCRAQ method suggested that in both season and annual scales, the decreasing runoff depth could be ascribed to variations of precipitation, evapotranspiration, and vegetation restoration. Specifically, precipitation explained 50.88%, 42.60%, -10.39%, -3.28%, and 31.26%, evapotranspiration explained 32.89%, 40.71%, 29.33%, 47.43%, and 42.64%, and vegetation explained 16.23%, 16.69%, 81.06%, 55.85%, and 26.10% of the decreasing mean runoff depth in the spring, summer, autumn, winter, and whole year, respectively. We concluded that precipitation and evapotranspiration were the key factors regulating the runoff variation in spring and summer, vegetation restoration played a major role affecting runoff variation in autumn and winter, and evapotranspiration contributed the most on an annual scale. This study revealed runoff changes and the main driving factors at different time scales in the Pengchongjian watershed over the past 30 years; thus, provided a scientific basis for the basin rational allocation and management of water resources.