Water erosion is the most universally-distributed soil erosion type at present and does a lot of harm. Soil erosion is a chief cause of land degradation, which is also an important factor of eco-environment deterioration. The process of hill-slope water erosion depends on the interaction of rainfall, the erosion power of runoff and the ability of the soil to resist erosion. Forest vegetation participates in the soil erosion process, regulating and controlling water erosion with its peculiar hydro-ecological characteristics, changing the erosivity of rainfall and runoff to hill-slopes. Hydraulic characteristics of overland flow, such as flow velocity, flow depth and friction coefficients, etc., and their relationships have been studied widely on overland flow. However, few studies have been done examining inter-rill flow in vegetation-covered plots under rainfall conditions. This paper illuminates dynamic mechanics of each vertical part of forest vegetation influencing hill-slope water erosion, summarizes the research achievements and research progress that different scholars have made, and points out the problems and developing directions in this field that currently exist: Forest vegetation canopy redistributes rainfall and changes the characteristics of rainfall such as the raindrop size distribution, rainfall velocity and raindrop kinetic energy. These have an influence on hydraulic characteristics of overland flow and can change the mechanism of soil erosion on hill-slope. Trunks prevent and disperse runoff and increase the surface runoff resistance coefficient. Runoff flowing around trunks has both positive and negative functions on hill-slope water erosion. Within a certain planting density, forest vegetation can decrease initial velocity of sediment and increase hill-slope water erosion, so reasonable planting density should be chosen to decrease water erosion. Runoff flows in and through the litter layer and infiltrates into the soil, similar to the flow in porous media. Litter has a different flow state for different degrees of decomposition, so it is necessary to do further research to get the flow mechanism by percolation theory. The forest vegetation root system gradually improves the inherent characteristics, structure stability and infiltration of the soil, and strengthens the anti-erosion ability. Research on the role of plant root system to the slope soil erosion is a new field. Anti-erosion ability of soil should be studied from the aspects of soil mechanics and plant roots influencing soil mechanic characteristics.