Colloids are among the most active components in agricultural soils and their large specific surface area and surface charge make them highly susceptible to pollutant adsorption. During the water cycle in the saturated-unsaturated zone, colloids can carry contaminants through porous media via size exclusion effects, leading to accelerated diffusion and significant impacts on groundwater quality. This paper reviews the size exclusion mechanisms of colloid transport in porous media, systematically analyses the key factors influencing the size exclusion effects of colloid transport, and summarizes the mathematical models used to describe the size exclusion effect of colloid transport. Finally, it explores high-precision tracer methods for characterizing colloid transport behavior and discusses strategies for constructing mathematical models to depict size exclusion effects in colloid transport. This study holds significant implications for deepening the understanding of environmental effects associated with colloid transport in the saturated-unsaturated zone water cycle and guiding the prevention and control of groundwater pollution in farmlands.