ABSTRACTA large area of underground spaces form after the extraction of underground mineral resources, in which coal mining leads to billions of cubic meters of underground spaces every year. In China, there are 2,469 coal mines exist within 50 km (straight line distance) of the cities, and the annual volume of the goaf in the coal mines is 442 million m3. However, most of these spaces are filled caved rock fragmentation and can hardly be retained and used. To fill this research gap, an innovative scientific conception is proposed in this paper. High-water and quick-setting materials (HWQSMs) are used to build backfill bodies rapidly in mined-out spaces during longwall mining to retain a large amount of underground spaces. Three different methods of building cubic or cylindrical backfill bodies in the mining goaf are introduced. An integration craft and the potential backfilling materials for the construction of the underground space are described in detail. Technical and economical rationality of the proposed conception is evaluated. Further research prospects and the problems that need to be overcome to achieve this goal are discussed. The results shows that compressive strength reaches 10.5 MPa when the HWQSM specimen has a water to HWQSM ratio of 1.5 and the specimen sustains an obvious residual load-bearing capacity. Hence the backfill body will not fail instantaneously in the process of roof supporting. The average unitary underground space value is estimated to be 2,838.89 Yuan per m3, which has great economic value.