Abstract: Sediment transport capacity of overland flow, which is the maximum equilibrium sediment load that a flow can transport, is a key concept for developing process-based soil erosion models because it plays a pivotal role to determine soil detachment rate and sediment transport. Few studies have been conducted to investigate the mechanism of sediment transport capacity by overland flow on steep slopes. In this paper, based on the systematic definition of sediment transport capacity of overland flow, the influences and their mechanisms of hydraulic properties of overland flow and sediment characteristics on sediment transport capacity were deeply analyzed. Sediment transport capacity of overland flow has great spatial and temporal variations due to the comprehensive effects of precipitation, topography, soil, vegetation, land use, sealing formation, rill development, and gravel covering. Therefore, it is very difficult to measure sediment transport capacity of overland flow directly in field conditions. The commonly used methods to determine sediment transport capacity of overland flow can be categorized into the modification from river sediment transport capacity formula, the rational method, the model simulation, and the direct measurement. The advantages and disadvantages of each method were compared and analyzed. Further studies should be focus on the measuring methodology and techniques for sediment transport capacity of overland flow, the hydro-dynamic mechanism of sediment transport process, sediment transport capacities of typical soil types, transport capacity formula development and performance evaluation, and sediment transport capacity of overland flow under changeable conditions. It is helpful to summarize the conclusions of different studies, promote the research on sediment transport capacity of overland flow, and improve the prediction precision of the process-based soil erosion models for better service to water and soil conservation planning and design.