The invention relates to a mined out space dam-type filling method; after an overhand mining working surface is pushed forward 3-5m, a baffle is arranged behind a cut top row single-body hydraulic support post in a spanning way; flexible waterproof cloth is attached on baffle and is laid and extended towards a bottom plate and two lanes of the working surface, so that a separated mined out space is formed; high-water material is filled into the separated mined out space by grouting equipment; after the high-water material is solidified, the baffle and the single-body hydraulic support post are disassembled, and one-time filling can be finished; along with the advance of a coal mining working surface, the steps are repeatedly carried out for completing the filling for a plurality of times; after the coal mining working surface finishes mining, all devices of the working surface are disassembled, a closed wall is built between a transportation roadway and a track roadway, and finally, top tight filling is carried out to complete the filling operation of the whole mined out space. As a grouting pipe is used for flood irrigation of the separated mined out space, the filling liquid level is lifted, slurry can be in top tight filling rapidly, filling is dense, the effect is visualized, and the method is simple. Therefore, the mined out space dam-type filling method is applicable to the overhand mining working surface with smaller coal seam dip angle or higher mining height.