Abstract: The production of coal gasification slag in our country is enormous and traditionally disposed of mainly by stacking and landfilling. Conventional disposal methods accelerate resource depletion and incur expensive post-treatment costs, contradicting China's policies of reducing, reusing, and harmlessly treating solid waste. Coal gasification slag is the waste discharged from coal in the gasifier after high-temperature gasification, containing a certain amount of unburned carbon, which can serve as an important raw material for preparing activated carbon. This study takes coal gasification fine slag as the raw material, and after flotation to obtain fine carbon, a chemical activation method is used to prepare activated carbon. By investigating conditions such as alkali-to-carbon ratio, activation time, and activation temperature, the optimal preparation process conditions are determined to be an alkali-to-carbon ratio of 1, activation time of 60 min, and activation temperature of 900 °C. Under these conditions, the specific surface area of the obtained activated carbon is 326.77 m2/g, the CO2 adsorption capacity at 298 K is 20.63 cm3/g, and the CO2/N2 (15/85) adsorption selectivity is 38.5, showing significant improvement compared to flotation fine carbon. The samples were characterized for structure and performance by low-temperature nitrogen adsorption (BET), powder X-ray diffraction (PXRD), scanning electron microscopy (SEM), Fourier transform infrared spectroscopy (FTIR), CO2/N2 adsorption, and breakthrough testing. The results indicate that the activated carbon obtained after activation has a more abundant porous environment, especially with a significant increase in micropores and mesopores, which significantly enhances its CO2 adsorption performance compared to the raw material, making it effectively applicable for CO2 capture.