In order to explore the migration law of lead ions in unsaturated loess under freeze-thaw cycle condition, this paper conducted adsorption experiment and soil column experiment on lead ion migration under freeze-thaw cycle condition. Based on the principle of heat mass conservation, a coupled mathematical model for pollutant migration in unsaturated soil was established. At the same time, the COMSOL multiphysics simulation software was used to numerically solve the model. The reliability of the coupled model was evaluated in combination with soil column experiment, and the variation laws of various physical quantities were explored. Research has shown that under the freeze-thaw cycling condition, the temperature change of the soil column lags behind the change in cycling temperature. Although freeze-thaw cycling significantly reduces soil permeability and sharply decreases liquid water flux, the decrease in liquid water flux does not completely hinder the migration of pollutants to the freezing zone. Ionic diffusion can also cause pollutants to appear in the frozen zone of the soil, leading to an increase in the adsorption concentration of pollutants in the frozen zone. At 96 hours, the mass concentration of pollutants at the top of the soil column reached 0.204 g/L, and the adsorption concentration of pollutants reached 3.62×10^-7 kg/kg; Under freeze-thaw cycle condition, pollutant crystallization occurs at the lower part of the soil column, with a maximum crystal volume content of around 4%. At the same time, the expansion caused by pollutant crystallization accounts for about 9% of the total soil displacement, which is much smaller than the expansion displacement of ice crystals. The negative temperature gradient decreases, the solubility of pollutants increases, and some of the precipitated pollutant crystals dissolve again. Some of the ice crystals also melt again, and the combination of the two causes the displacement of the soil column vertex to slowly increase in a wave like manner.