Abstract: In view of the problem that water inrush is easy to occur in the mining of coal seam under aquifer, taking a mine in Shaanxi Province as an engineering background, this paper proposes a paste filling technology behind transitional support in fully mechanized top-coal caving mining, expounds its technical principle and technological process, and analyzes the development characteristics of water-conducting fracture zone in back-support filling mining and the variation laws of water inflow in working face by means of numerical simulation and field observation. The numerical simulation results show that the water-conducting fracture zone of the working face develops into a rectangle with a maximum height of 67.2 m after the backfilling technology is adopted. The field observation results show that the filling rate after support can reach 90%, the water inflow of working face decreases significantly from 220 m³/h to 90 m³/h, and the maximum development height of water-conducting fracture zone is 76.8 m after 200 m advance of working face. The research results show that the technology can effectively reduce the height of water-conducting fissures in overburden and realize safe mining of extra-thick coal seam under aquifer.