Loess is characterized by low resistance to hydraulic erosion and a propensity to release soil colloids. To investigate the influence characteristics and mechanisms of loess colloids with varying particle sizes on the adsorption, desorption, and transport of petroleum contaminants, and to reveal the environmental behavior of loess colloids in petroleum contaminant migration. Loess colloids were fractionated into three size ranges: below 2 μm, 2 ~ 5 μm, and 5 ~ 10 μm. Temperature-controlled oscillatory column experiments were employed to investigate the mechanisms governing the adsorption, release, and migration of petroleum contaminants in the presence of these colloids. The results show that diesel adsorption across all colloidal fractions is well described by pseudo- second-order kinetics and the Henry isotherm model, exhibiting rapid kinetics with equilibrium attained within approximately 30 minutes. Adsorption capacity decreases sequentially as follows: 5 ~ 10 μm > 2 ~ 5 μm > below 2 μm. Desorption also conforms to pseudo-second-order kinetics, but requires a significantly longer equilibrium time of 900 minutes. Conversely, desorption capacity increases with decreasing particle size: below 2 μm > 2 ~ 5 μm > 5 ~ 10 μm. Particle size exerts a marked influence on desorption isotherm patterns: desorption from the 2 ~ 5 μm and 5 ~ 10 μm fractions follow Henry-type behavior, while desorption from the below 2 μm fraction is best described by the Freundlich model. The superior adsorption capacity of the 2 ~ 5 μm and 5 ~ 10 μm colloids is attributed to their larger average pore diameter and higher density of reactive surface functional groups, which facilitate the penetration and sorption of diesel molecules. Furthermore, all colloidal fractions inhibit diesel migration, with the inhibitory effect being inversely proportional to particle size, indicating a stronger retentive capacity for smaller colloids.