【Objective】Colloid is an active component, exsiting extensively in soil, so how it affects adsorption and migration of sulfadiazine in soil is a scientific issue that is worth further exploring.【Method】Taking Neutral Ludox as extraneous colloid, this work quantitatively analyzed effects of ionic intensity（0.001, 0.003, 0.005, 0.010 mol L^-1CaCl₂）on the adsorption and migration of sulfadiazine in soil, 6.38±0.03 in pH, where colloid and sulfadiazine coexisted through indoor batch experiments and soil column effluent experiments.【Result】Results showed that the colloid inhibited the adsorption of sulfadiazine in the soil when the ion intensity was 0.001 or 0.003 mol L^-1, but the effect of the colloid inhibiting sulfadiazine adsorption in the soil with ion intensity being 0.005 or 0.01 0 mol L^-1CaCl₂ was effected by the initial concentration of sulfadiazine. Freundlich model could well describe the adsorption process of sulfadiazine in the soil with R² being 0.958~0.997. The breakthrough curve of vertical transport of sulfadiazine in the saturated condition was delayed in all conditions mainly due to the existence of different forms of sulfadiazine. Neutral ion form of sulfadiazine was adsorbed on soil organic matter through hydrophobic interaction and sulfadiazine in negative ion form was adsorbed on the soil through bonding action. When concentration of sulfadiazine was 30 mg L^-1, however, in soil higher in ion intensity, the colloid added could inhibit migration of the sulfadiazine due to the dielectric layer was compressed, forming flocculation that absorbed a certain amount of sulfadiazine, while in soil lower in ion intensity, colloid outflowed, entirely or partly, thus impoving sulfadiazine migration.At 0.010 mol L^-1 CaCl₂in ionic intensity, the outflow of sulfadiazine from the soil column was advanced, the relative concentration of sulfadiazine peaked higher, and the outflow lasted longer without colloid addition. However, once colloid was added, the relative concentration of sulfadiazine peaked low in value and it was hard for sulfadiazine to outflow. At 0.005 mol L^-1CaCl₂ in ion intensity, the outflow of sulfadiazine was earlier and the relative concentration of sulfadiazine peaked higher than that without colloid addition. At 0.001 mol L^-1 CaCl₂ in ion intensity, relative concentration of sulfadiazine peaked higher than that without colloid addition, and it was more difficult for sulfadiazineto to get eluted.【Conclusion】Generally speaking, in soils with colloid existing and lower in ion strength, the addition of colloidal inhibits the adsorption of sulfadiazine and hence promotes the migration of sulfadiazine, while in soils higher in ion strength, it promotes the adsorption of sulfadiazine and hence inhibits the migration of sulfadiazine. All the findings in this study may be of some guiding significance for controlling the migration of sulfonamides in soil.