Coal gasification slag and fly ash are solid wastes produced in the process of coal resource utilization, which can be used in the field of alkali-activation. In this paper, by studying the properties of coal gasification coarse slag, coal gasification coarse slag was modified with fly ash, and the coal gasification coarse slag-fly ash based geopolymer was prepared by alkali-activation technology, and the properties of prepared products were studied. The results show that adding fly ash into the system significantly improves its mechanical properties. When the mass fraction of fly ash is 30%, the 28 d compressive strength of the sample is the highest, which is 44.5 MPa. In addition, through the phase analysis and micro morphology characterization of the sample, it is found that the amorphous product of the sample is mainly N(C)-A-S-H gel, which can form an interconnected spatial network structure and has strong bonding ability, which is the main reason for the high strength of the sample material.