Abstract: In order to further improve the recovery rate of the coal resources and to have a safety mining, with the analysis on the hydrological conditions of the bel t coal pillars left under the previous building structures and under the water bodies, the paper proposed the technical idea with the high water material backfill to replac e the coal pillars for the secondary mining.An analysis study was conducted on the strength and safety of the backfill material based on the engineering mechanics.The results showed that under the conditions of the backfill material strength over 2 MPa, the face width of the secondary mining about 20 m and the width of the backfill ma terial and the coal pillar about 25 m, the full mining and full backfill method could have a safety mining under the building structures or above the pressurized water stra ta