Abstract: According to engineering conditions of 2334 fully mechanized coal face,using the software UDEC to simulate the hard roof mining activities under different thickness,the regularity of stress evolution and energy conversion when the hard conglomerate roof fractured revealing for revealing The results show that,with the increasing of main roof thickness,the first weighting interval and the periodic weighting interval are raising too.The main roof fracture energy was reduced by the measures of increasing the energy consumption in the process of fracturing,decreasing energy input efficiency from external load to the main roof and lessening main roof gravitation energy.