Abstract: In-situ thermal desorption (ISTD) is a remediation technology that has emerged and been applied on a large scale in China in recent years. In order to clarify the concentration levels of pollutants in different media during remediation and to carry out verification scientifically after remediation, the ISTD remediation project of a chlorinated hydrocarbon contaminated site was used as a case study. At the end of the remediation heating cycle into the cooling stage, soil and soil gas profile samples were collected for testing and analysis to finely identify the concentration levels and spatial distribution characteristics of pollutants in soil and soil gas. The effects of pollutant physicochemical properties, stratigraphic structure, especially the low permeability layer on gas migration and pollution distribution were analyzed. In view of this, the verification of soil remediation and the secondary pollution prevention and control after ISTD were proposed. The results showed that: (1) Only 1% of soil samples were detected of chlorinated volatile organic pollutants in trace amounts in this case, and all samples met the remediation target values. (2) Pollutants in soil gas were detected at different concentrations, among which trichloroethylene was detected at the highest concentration of 2310 μg/m³, which has a potential health risk. (3) The low permeable layer has a blocking effect on the migration of gas-phase pollutants, and high concentrations of pollutants accumulated under the surface cement layer. (4) Trichloroethylene, tetrachloroethylene and cis-1,2-dichloroethylene have low boiling points, low soil organic carbon partition coefficients (K_OC), high vertical migration efficiency, and the highest gas-phase pollutant concentrations in the top layer. Hexachlorobutadiene has a high relative boiling point and high K_OC, and the maximum value occurs in the deep low-permeability stratum. The study showed that the ISTD technology has a good removal effect for high concentration of chlorinated hydrocarbon pollution, but the contaminants were still detected in soil gas after the soil reached the standard in this case study. Therefore, it is recommended that both multi-media verification of soil and soil gas should be conducted after ISTD remediation for a comprehensive assessment. The identification of low permeability layers in the ISTD area should be enhanced to optimize the gas phase extraction scheme.