Abstract: There are a lot of difficult coal resources in the residual mining areas in China. The shallow residual coal reserves are considerable and have great mining value. Safe recovery can help the continuous supply of coal resources in China. Moreover, the long-term retention of residual coal is easy to spontaneous combustion, which threatens environmental safety and is difficult to dispose of. Consequently, adhering closely to the academic ideas of green mining and scientific mining, integrating backfilling mining with underground coal gasification (UCG), a technical approach is proposed for the critical domains backfilling synergistic UCG for remining shallow difficult residual coals. Through the critical domains whole process identification, backfill materials optimization and structural design, UCG space reconstruction, UCG for residual coal resources, and remining area monitoring as well as backfill comprehensive control, the safe, low-carbon, and efficient extraction of residual coal resources can be achieved by the technology system. Four key scientific issues require resolution in critical domains backfilling synergistic UCG for remining shallow difficult residual coals: the instability and failure characteristics of coal and rocks in the residual coal occurrence space and the discrimination principle of critical domains, the thermal damage and high temperature resistance mechanism of backfill materials, the instability mechanism and bearing-seepage features of the coal, rock, and backfill in UCG, and backfill structure design and parameter optimization criteria. Four key technologies faced by critical domains backfilling synergistic UCG for remining shallow difficult residual coals include the multi-parameter identification technology for critical domains in the residual mining areas, the optimization technology of backfill materials, the chain catastrophe simulation technology of combustion cavity, and the parameter control technology of “backfill-gasification” in critical domains. The research contents of critical domains backfilling synergistic UCG for remining shallow difficult residual coals are as follows: the precise identification of critical domains and the scientific determination of backfill positions, the backfill material optimization and structural design, the instability characteristics and catastrophe mechanism of the distributed combustion cavity, and the rock strata stability control method of critical domains backfilling synergistic UCG. This technology is anticipated to realize the recovery of shallow difficult residual coals and the reuse of mine solid waste simultaneously. This integrated approach can not only facilitate the advancement of UCG and rock strata control technologies but also ensures energy supply security for the nation.