Modified coal gasification slag (MCGS) adsorption material was prepared by hydrothermal method, by NaOH activation to remove Cd2+ from aqueous solution. Due to the low Cd2+ adsorption capacity of coal gasification slag (CGS), the modification conditions were optimized by Box-Behnken response surface method. X-ray diffraction (XRD) analysis revealed the presence of amorphous "steamed bread" SiO2 and residual carbon peaks in the range of 15°~30° in the coal gasification slag (CGS). These peaks had broad diffraction patterns. Fourier transform infrared (FTIR) spectroscopy showed that MCGS contained stretching and bending vibration peaks for the Si-O-T bond (where T is either Al or Si), indicating that the modified Si-O-Si bond had been broken and the vibration peak had increased. Scanning electron microscopy (SEM) images showed that the CGS mainly consisted of microbeads with smooth surfaces and flocculent or flaky blocks. The surface of the MCGS had an abundance of pores, with a specific surface area, pore volume, and pore diameter of 255.08 m2/g, 0.24 cm3/g, and 3.72 nm, respectively. The modification results showed that the best reaction conditions of MCGS were basicity 6.20%~8.10%, temperature 102~108℃, and time 138~192 min, and the temperature was the greatest influence on Cd2+ adsorption performance of MCGS. The adsorption results showed that when the concentration of Cd2+ was 50 mg/L and the dosage of MCGS was 0.10 g, the saturated adsorption capacity of Cd2+ was 13.96 mg/g; when the concentration of Cd2+ was 40 mg/L and the dosage of MCGS was 0.20 g, the removal rate of Cd2+ was 98.08%. The adsorption of Cd2+ on modified coal gasification slag follows a quasi second-order kinetic model, indicating that the adsorption of Cd2+ by MCGS was mainly chemical adsorption. The isothermal adsorption of Cd2+ onto the modified slag can be described well by the Langmuir model, indicating a monolayer adsorption process. This study can provide a theoretical basis for the treatment of Cd2+ wastewater with alkali-modified coal gasification slag.