Abstract: To solve the problem of gas overrun in return airway and upper corner under "U+L" type ventilation roadway at 3405 fully mechanized coal face, this paper established physical model and mathematical model with "U+L" and "U+U" type ventilation system based on the actual situation. And by comparing the two kinds of ventilation system simulation results, gas distribution laws of goaf were analyzed along the direction of working face advancing, face width and vertical direction. Numerical simulation and the gas concentration measurement of 34052 and 34053 return airways and upper corner showed that the type of "U+U" ventilation system effectively ensured low gas concentration in return airway and upper corner and the safety mining at working face.