Abstract: The residual void ratio of the goaf is an important technique parameter for stability evaluation of foundations above the goaf and the design of the backfill grouting scheme. In order to determine the reasonable value of the residual void ratio in the underground knife pillar goaf below the new substation, the discrete element software of UDEC was used to simulate the collapse characteristics of the overburden for the 2# coal knife pillar goaf, and the simulated caving height was 9.0m. Considering the bulking effect of caved gangue, the residual volume of the block within the caving range of the gob is obtained by using a self-defined fish function, and the average residual void ratio was 0.233 in the caving zone of the 2# coal knife pillar gob. Based on the simulated residual void ratio, the backfilling grouting volume of the goaf under the substation was predicted to be 25267m3, and the deviation between the predicted result and the actual injection volume was only 5.4%. Finally, it can be concluded that the simulated residual void ratio can represent residual void degree in the caving area of the goaf left by knife pillar method, and this method can provide technical reference for goaf treatment by backfill grouting.