The stabilizing effects of improved bentonite-diatomite-magnesia-humic acid composite curing agent on soil available Cd and its influence on soil physicochemical properties were investigated, and the related mechanisms were also studied by adopting FTIR, SEM-EDS and BET analyses. Results showed that after 70-day treatment, the concentration of available Cd in soil, which was initially 0.0836 mg/kg, decreased significantly. The stabilizing efficiencies of different composite curing agents were in an order of 10 g/kg curing agent +0.4 g/kg humic acid > 10 g/kg curing agent > 5 g/kg curing agent + 0.4g/kg humic acid > 5g/kg curing agent > 0.4 g/kg humic acid. In particular, the stabilizing efficiency reached 60.2% for 10 g/kg curing agent + 0.4 g/kg humic acid treatment. SEM-EDS and BET analyses revealed that the curing agent exhibited loose surface structure, the existence of ions on the surface, which included Na⁺, Ca²⁺, Mg²⁺, could contribute to chemical adsorption through ion exchange. Besides, functional groups such as quinone, alcohol, hydroxyl, carboxyl, carbonyl and C=C bond in humic acid could coordinate with Cd²⁺. Chemical absorption is the main reason to reduce the availability of soil Cd.