Abstract: In terms of fully mechanized mining with large mining height and dip angle,through theoretical analysis, simulation and field measurement,the disaster mechanism of surrounding rock caused by the size effect of working face width was analyzed. The study found that the coordination of immediate roof and main roof movement in the large dip angle coal seam increases the difficulty of roof control. The mine pressure behavior characteristic shows a typical “weak-strong-weak” distribution. The working face roof structure along the tendency was divided into sliding section and buckling section. The sliding section had the strongest regional strata pressure,and also was the main control factor of mine pressure behavior. According to the occurrence conditions of coal seam with large dip angle,there is a “critical size” in the length of working face. In addition to meeting the weight of direct roof rock orientation and part of the roof load,the support strength must also meet the additional load generated by top slippage. And the prevention of separa- tion at buckling section is the main basis to determine support strength.