Abstract: The stability system of support and surrounding rock is broken when the rib spalling occurs at a fully-mecha- nized top coal caving face in soft seam,which further causes a disaster. Comprehensive research methods of model building,numerical simulation and theoretical analysis have been used to study the mechanical mechanism,morpholog- ical features and disaster from different regions of a fully-mechanized top coal caving face. Results show that coal wall broken behavior is mainly a shear and slippage style. Rib spalling figurations show as an irregular four pyramid mass, which may occur secondly and even constantly. Rib spalling is easily taken place in the middle and upper sections of working face. After these behaviors occur,the support system is out of control with the roof and floor deformation under the circumstance of worsen repetition process from rib spalling to roof falling,and then secondly to rib spalling. The study concludes that the disaster mechanism of surrounding rock is the failure process from rib spalling to roof falling, to support instability,eventually to floor-support separation. On the basis of safety production demands of No. 1123 working face in Changshanzi Coal Mine,some relative control measures have been taken according to the results in this paper.