Thinning can significantly change the biomass and soil properties of forest ecosystems, thus affecting the development of overland flow. To estimate the effects of thinning intensity, overland flow plots were established in a Chinese fir plantation in the Huitong State Ecosystem Research Station. The rainfall intensity results showed that, in the absence of downpours and super rainstorms, the overland flow after thinning was less than that in the control plots. Additionally, the overland flow increased with increasing rainfall intensity. However, during downpours and super rainstorms, there were no significant differences between the thinned and control states. Therefore, we suggest that thinning could significantly reduce overland flow only in the absence of downpours and super rainstorms. Subsequently, we investigated the monthly changes in rainfall and overland flow. During the dry season (from November to March), low precipitation (light and moderate rain) was prominent, and the corresponding overland flow was very low. However, during the rainy season (from April to October), heavy rain was dominant, resulting in a greater overland flow. Although the total rainfall in May was greater than that in July, the corresponding overland flow was lower; this may be explained by increased frequency of super rainstorms in July, directly leading to a greater overland flow. Consequently, it can be concluded that individual rainfall events rather than total rainfall, was the primary reason for the formation of overland flow in the forest ecological system. Moreover, the overland flow decreased with increasing the thinning intensity, and a thinning intensity of 30% was the most effective in reducing the overland flow. The results of an analysis of vegetation and soil properties under different thinning intensities indicated that thinning could significantly increase the growth of undergrowth vegetation as well as improve soil fertility. Thus, the undergrowth and litter biomass, coverage of the undergrowth plant layer, and plant species richness increased to 786.2 kg/hm², 786.2 kg/hm², 7.7%, and 0.56, respectively, with 15% thinning intensity, whereas the values increased to 1658.9 kg/hm², 1796.4 kg/hm², 14.9%, and 0.94, respectively, with 30% thinning. Thinning promotes faster growth of undergrowth plants by increasing light availability throughout the vegetation structure. Additionally, the noncapillary porosity, capillary porosity, average particle size of the water stability soil aggregate (APSWSSA), organic carbon, rapidly available potassium, rapidly available phosphorus, and rapidly available nitrogen under 15% thinning intensity were, respectively, 1.22, 1.04, 1.28, 1.33, 1.10, 1.46, and 1.35 times the control values, whereas they were 1.33, 1.10, 1.81, 1.90, 1.24, 1.92, and 2.24 times the control values under 30% thinning intensity. The soil bulk density decreased with increasing thinning intensity, from 1.27 to 1.11, and to 1.02 under the control, 15%, and 30% thinning conditions, respectively. The analysis of overland flow, vegetation, and soil factors indicated that leaf litter, coverage, noncapillary porosity, and particle size of water-stable aggregates were significantly negatively correlated with overland flow. In contrast, soil bulk density was significantly positively correlated. Thinning improved soil and water conservation by changing the abovementioned factors. Additionally, under the experimental conditions, 30% thinning intensity had a more significant effect on reducing overland flow.