ABSTRACT: This paper makes research on the water-blocking effect of bearing structure in main roof on the theoretical basis of equilibrium structure formation along the dip direction of steeply dipping coal seam. By establishing the relation between the force of bearing structure in main roof and pore pressure in WangJiaShan coal seam, it is concluded that the bearing structure of main roof begins to prevent from water raining when mining depth is more than 238.52 m. It is also concluded that the seepage flow begins to decrease in working face when the mining depth is more than 235 m by analyzing the relations between the seepage flow in working face and mining depth. The monitoring result on site is consistent with the theoretical calculation results. It is shown by numerical simulation results that the rainwater seepage disappears in some areas while the mining depth is more than 200m, which can explain the water-blocking mechanism of the bearing structure of main roof. It is helpful for preventing the infiltration of rain water. 1 INTRODUCTION The seams with angle between 35° and 55° are called steeply dipping seams. It is difficult for these seams to be mined. The reserves possession of steeply dipping seam is about 15%-20% in China (Shi & Gao 2003). The product of steeply dipping seams accounts for 10% of the total coal product in China. The steeply dipping seams mainly distribute in Xinjiang, Sichuan, Gansu, Fujian, Guangdong, Inner Mongolia, Northeast China, etc (Wu et al. 2000). The steeply dipping seams accounts for a large proportion in Gansu provinces such as WangJiaShan Coal Mine. In Gansu province, coal seams are covered with the loess foothills areas. In rainy seasons, it is easy for rain to seepage into subsidence area along cracks. Then it will result in flooding accidents. It is found by on-site investigation and theoretical study that the cracked overlying rocks above the mining face gradually compact tightly and form bearing structure with the increasing mining depth. The bearing structure can prevent from surface water to some extent. This paper will study the water-blocking mechanism of bearing structure based on the mechanical model along dip direction of long wall face in steeply dipping seam proposed by Professor Shi Pingwu and Wu Yongping (Shi & Chen 1990). It will be important for treatment of water damage.