In order to improve the poor grading, loose overall structure and low strength of silt as subgrade material, the composite curing agent was prepared with red mud (RM), carbide slag (CS) and desulphurized gypsum (DG) as raw materials, and the mix ratio optimization of composite cured soil was carried out in this paper. The reasonable content range of red mud, carbide slag and desulfurized gypsum cured soil was determined by single factor test, and the mix ratio of composite cured soil was optimized by Box-Behnhen response surface method. A second-order regression model was established with 7 and 28 d unconfined compressive strength (UCS) as response values. The results show that the significant degree of influence of single factor on the unconfined compressive strength of cured soil at 7 and 28 d is carbide slag, red mud and desulphurized gypsum. The influences of two factors on the unconfined compressive strength of cured soil at 7 d is in the order of red mud and carbide slag, red mud and desulphurized gypsum, carbide slag and desulphurized gypsum, and the influence on the unconfined compressive strength at 28 d is in the order of red mud and carbide slag, carbide slag desulphurized gypsum, red mud and desulphurized gypsum. The optimal mix ratio of composite cured soil is 15.84% (mass fraction) red mud, 14.32% (mass fraction) carbide slag and 7.32% (mass fraction) desulphurized gypsum. The error range of the measured and predicted response values is controlled within 3%, which proves the accuracy and scientificity of the response surface method in optimizing the mix ratio of composite cured soil. The main products of cured soil are calcium silicate hydrate (C-S-H) and calcium aluminate hydrate (C-A-H) with gelling effect, and the resulting ettringite (AFt) is interspersed between the gel and soil particles to enhance the strength of soil.