The massive stockpiling of coal gasification slag poses significant environmental hazards, necessitating urgent resource utilization. When used in cement-based materials, it can partially replace cement, thereby reducing carbon emissions. This paper employs different pretreatment methods to enhance the hydration activity of coal gasification slag and compares modification techniques using various evaluation methods. The results indicate that, in terms of evaluation methods, the Rietveld whole-pattern fitting method, ion leaching method, and quality loss method can rapidly assess pozzolanic activity; the combined application of these methods can simultaneously ensure high accuracy and low time consumption. Regarding modification techniques, the modification methods adopted in this study can alter the amorphous composition, ion dissolution, and reactivity of coal gasification slag. Under the experimental conditions of this study, alkali dissolution and CaCl₂ modification can effectively enhance pozzolanic activity, resulting in stronger mechanical properties. Specifically, the compressive strength growth rates of cement with coal gasification slag modified by NaOH at 7 d and 28 d can reach 13.5% and 6.5%, respectively, while those modified by CaCl₂ can reach 9.0% and 13.1% at 7 d and 28 d, respectively, demonstrating significant effects. Therefore, these methods can be considered as viable means for modifying coal gasification slag to improve its resource utilization efficiency.