Abstract: Ⅱ5112 working face of RenLou Coal Mine in Wanbei Coal and Electricity Corporation was under roof water conditions,when it reached the geologicstructure area,a large range of support break-off accidents occured.To this problem,the paper use the field measurement and theoretical analysis to find out the reasons about support break-off,as follows:the thickness increase of roof siltstone,the complex geological conditions,the low mining height(1.2-1.5m),the ineffective management,the unreasonable set pressure of safety valves,and the slow pushing velocity.By means of the theory and practice, it put forwards a series of measures that managers in coalmine should strengthen the management in face,develop the mine pressure and water level monitoring early-warning,and carray out the reasonable control of the working face mining height.

Key words: support break-off, fully-mechanized face, prevention measures