Abstract: In order to accurately assess the threat level of the flying gangues to people or equipment during the mining of the steeply dipping seam, the total kinetic energy of the flying gangue was used as the basis for determining the magnitude of the threat level, and model of flying gangues movement was establish by Rocfall in the steeply dipping seam. The six factors-five levels including flying gangue’s mass, initial velocity, working face length, dip angle of coal seam and collision recovery coefficient were selected for the standard orthogonal experiment, and the order of the sensitivity of the six factors was obtained by the orthogonal experimental method. According to the sorting and the actual situation of the working face,the major factors affecting the dynamic threat level of the flying gangues are the quality, speed and distance moving along the working face. Dynamic Bayesian network model was set up by adopting the Genie, the dynamic probability of threat level was obtained during the whole process of flying gangues movement by simulation. The simulation results show that the threat level of the flying gangues increases sharply in the initial stage of fly gangue movement. As the time slice increases, the threat level tends to be stable in the middle and end stages. In order to reduce the damage caused by the flying gangues of the working face, the protective devices can be set up at the initial position of flying gangues movement and the position with higher threat level to achieve the purpose of reducing flying gangues threat.