Abstract: In order to solve the collapse problem of underground coal gasification and reduce the dissolution of heavy metal cadmium from underground coal gasification slag, the high water filling material of coal gasification slag was prepared by the solution polymerization method using coal gasification slag powder, silane coupling agent, methylacrylamide, methylbisacrylamide, and triethanolamine as raw materials. Then, the high water filling material of coal gasification slag was filled into the underground coal gasification slag. The phase, micromorphology, and functional groups of high water filling material for coal gasification slag were studied by XRD, SEM and FTIR. The results showed that when the amount of slag powder is 4 phr, the uniaxial compressive strength and residual strength of high water filling materials can reach 0.57 MPa and 0.41 MPa, respectively, and the shrinkage rate within 35　d is 10.5%. After filling high water filling material of gasification slag into underground gasification slag, the amount of cadmium ion leaching in the backfill decreases by 70.1% in the water environment with pH=5.