In order to analyze the mechanism of the dynamic disaster caused by underground mining in Yanzhou Mining Area,taking a vertical section crossing the Damachang fault as study object the dynamic disaster was analyzed using numerical simulation method according to the practical excavating sequence.The main causes of the dynamic disasters were identified as the yield of the 15 m security coal pillar,and the excavation in an "isolated island" coal mass.A displacement rotation field was found near the destructed zone.The mechanical cause of its formation is discussed simply in this paper.