Coal gangue separation is of key important for green mining. Underground pneumatic separation of coal and gangue based on the machine vision system is proposed in this study. This method has the potential to be used in coal mining due to the real time control and simple work principle. Here, the theoretical pneumatic separation distance formula of coal and gangue is derived based on energy conservation principle. Then hard-sphere model is applied to simulate the particle motion pattern in a three-dimensional model with a pressure-velocity boundary, and key factors affecting coal gangue pneumatic separation for large diameter (>= 50 mm) are analyzed. An air-solid multiphase flow simulation was conducted to clarify all the three factors (airflow velocity u, the conveyor velocity v(0) height difference h(p) between conveyor belt and air nozzle) have great influence on separation distance. Especially, the optimal parameters for best separation distance are presented at u = 320 m/s,v(0) = 0.5 m/s,h(p) = 0.4 m. Besides, the theoretical pneumatic separation distance formula is corrected based on experiment data, the error between experiment value and calculate value of the separation distance is less than 15%. The corrected formula has important guiding significance and practical value for coal gangue pneumatic separation. (C) 2015 Elsevier B.V. All rights reserved.