Objective Subsurface flow, constituting a critical component of slope runoff, constitutes the theoretical basis of soil and water conservation projects. A systematic review can deepen the understanding of subsurface flow processes and their hydrological functionality.

Methods This article systematically reviews research advances in subsurface flow, summarizes its formation and development mechanisms, existing monitoring methodologies, and key influencing factors, and focuses on the driving effects of subsurface flow on soil erosion.

Results The formation and development of subsurface flow depend on the "rainfall-infiltration" at the surface, and its dynamics can be quantified through various research methods, but there are still significant limitations. The subsurface flow process is jointly regulated by soil properties, rainfall characteristics, and topographic/vegetation conditions, thereby exhibiting strong spatiotemporal variability. Excessively developed subsurface flow exhibits direct/indirect erosion effects: subsurface flow can directly detach and transport fine soil particles, expand pore networks, and increase the risk of gravitational erosion. Moreover, it can also exacerbate soil degradation through mechanisms including soil structure alteration, rapid induction of pore water pressure changes, and weakening of soil shear strength, and even flow out of the ground again in the form of return flow, thereby forming compound erosion effects with surface runoff.

Conclusions Research on subsurface flow has evolved from the early descriptive stage of hydrological phenomena to the quantitative mechanistic stage of multi-process coupling. It has refined the influences of environmental factors on subsurface flow processes and further elucidated hydrological processes and pathways in the Earth’s Critical Zone. Nevertheless, further research should deepen the understanding of the interactions among "subsurface flow-surface flow-groundwater" under the synergistic effects of multiple factors, as well as the associated erosion effects.