A statistic-mechanical model of coal damage under abutment pressure is presented in this paper based on the concept that micro-unit strength which eflects the internal defect of coal obeys the Weibull distribution. The model is built on the premise of the random distribution of internal defect in coal and the distribution characteristics of abutment pressure in the front of fullmechanized caving mining with large mining height in ultra thick coal seam. According to the model, the reasonable mining height was determined for caving mining with large height in thick seam under normal mining conditions. The relationship of reasonable mining height respectively with rib spalling degree, internal friction angle, cohesion of coal and the deformation of top coal was also analyzed. Finally, the model was proved to be rational by engineering example analysis.