Thickness of soil layer interacting with overland flow through turbulent mixing, convection, and diffusion, etc., and contributing chemicals to surface runoff during a rainfall event is defined as the depth of mixing zone. It is a useful value to simulate the micro-mechanisms and behavioral traits in the compound body of topsoil-water-solute. The backgrounds, practical applications, existing problems and future directions of the mixing zone dexpth were reviewed. Rain intensity and duration, infiltration rate, degree of slope, soil cover, and chemical solubility and adsorption characteristics are the main effects on the depth of mixing zone. Different notions and models of the mixing zone depth were successively proposed, such as uniform and complete mixing depth, uniform but incomplete or nonuniform and incomplete mixing depth, effective depth of interaction, effective depth of transport, and effective depth of transport by runoff. The available methods to determine it are isotope or other chemicals tracing, and model calibrations, etc. As a key parameter to model soil solute release and transport with surface runoff, and to predict nonpoint source pollution load, it is also a useful tool to evaluate soil nutrient validity. The more researches in the future should be conducted on large-scale simulation, some reactive solutes, and the influences of near-surface soil and water conditions, soil erosion and chemical reactions in soil on the depth of mixing zone. Furthermore, its models or modified value suiting to some regions, especially to mountainous regions with more serious problems of soil erosion, are suggested to replace the 1.0 cm value in forecasting Nonpoint Source Pollution(NPS) pollution load.