The No.4 mined-out area is characterized by large volume and irregular figure in Guangxi Beishan mining Co.Ltd.The detailed in-situ investigation of engineering geological conditions of the rock mass surrounding the mined-out area was carried out.Based on the data from the in-situ investigation,the quality classification of the surrounding rock mass was made with RMR classification system and Q classification system,and the relativity of results of rock mass quality classification was analyzed.This provided basal geological data for the comprehensive treatment engineering of the No.4 mined-out area.