Abstract: according to the unusual failure law of surrounding rock, the failure destabilizaty of coal pillar ,the failure of abandoned roadway and roof of workface seek when coal pillar is mining. Research shows that: when the size of cross-section of abandoned roadway is relatively small, the failure of abandoned roadway is is less affected by the working face and the failure of coal pillar is mainly affected by abutment pressure and is less affected by the existence of abandoned roadway. When the size of cross-section of the abandoned roadway is relatively big, the failure of roof of workface is mainly affected by stress concentration cause by abandoned roadway and the destabilization size of of coal pillar is relatively bigger which is determined by abutment pressure and stress concentration. Theoretical analysis shows that: the bigger of the size of abandoned roadway is ,the bigger of the destabilization size of the coal pillar is. Locate observation show that the stress on the coal pillar enlarge and the number of wall caving and nicking enlarge because of the decreasing of the width of the coal pillar.