Abstract: Clay soil with fine particles and low permeability affected the thermal desorption efficiency of clay soil contaminated with polycyclic aromatic hydrocarbons (PAHs). Therefore, the six conditioners (CaO, MgO, Al₂O₃, Fe₂O₃, K₂CO₃, and zeolite) were selected to improve the clay soil and the thermal desorption efficiency of the PAHs-contaminated clay soil in this study. The effects of six conditioners on the plasticity index, pH, cation exchange capacity, and particle size of the modified clay soil were investigated and the mechanisms of the effects of conditioners on the thermal desorption of the PAHs-contaminated clay soil were discussed. The results showed that: (1) The addition of six conditioners improved the thermal desorption efficiency of the PAHs-contaminated clay soil, effectively increased the removal of the total PAHs, and significantly reduced the residual concentrations of the excess benzo[a]anthracene (Baa) and benzo[a]pyrene (Bap). The highest removal rate of total PAHs was achieved up to 97.48% when 10 % CaO was added to the clay soil. (2) After adding 6 % CaO, MgO, and K₂CO₃ to the clay soil, the plasticity index of the clay soil decreased from 18.55% to 14.38%, 13.58%, and 15.25%, respectively. However, the addition of Al₂O₃, Fe₂O₃, and zeolite increased the plasticity index of the clay soil. (3) Comprehensively, the improvement of the clay soil by the conditioners was mainly attributed to the cation exchange and flocculation. The results showed that the conditioners changed the fine grain content and cation exchange capacity of the clay soil, which affected the plasticity index and improved the thermal desorption efficiency of PAHs. In summary, CaO, MgO and K₂CO₃ could be suitable conditioners. This study provides a theoretical basis for the practical application of thermal desorption technology in the remediation of the PAHs-contaminated clay soil.