Abstract: There are large-scale consecutive underground mined-out areas in Xiaowanggou iron mine，which seriously threatens the safety of highway transportation and subsidiary buildings of the mine.In order to eliminate the threat of mined out area to the roads and buildings，it is decided to backfill the wasted rocks into the collapse pit to control the expansion of the collapse pit.The FLAC3D numerical simulation was used to predict the caving time of the mine-out area，and simulated tests on the surface displacement development of the collapse pit filled with waste rocks or not were carried out.Finally，the caving process of the mined-out area and the variation of the caving height after the waste rocks were filled were monitored by the drilling monitoring process.The field practice suggests that the caving process can be effectively controlled by backfilling the collapse pit with the waste rocks.

Key words: Mined-out area, Caving, Drilling monitoring, Numerical simulation, Control technology