Abstract: At present, the accurate measurement of air volume in large-size roadway has become a difficult problem for mine ventilation system managers. In order to solve this problem, firstly, with the help of the turbulence simulation function of COMSOL Multiphysics, taking the common rectangular roadway and semi-circular arch roadway in coal mines as examples, the distribution of instantaneous wind speed at each point in the roadway is simulated when the inlet wind speed is 0.25 m/s, 2 m/s, 6 m/s and 12 m/s. It is found that the position of the instantaneous wind speed equal to the average wind speed in the tunnel is only related to the geometric characteristics of the tunnel, and will not change with the change of the tunnel inlet wind speed, which verifies the existence of the “key ring”. Then the instantaneous wind speed of each point on the two-dimensional line 1.8 m away from the floor in the roadway is calculated, and the position where the instantaneous wind speed is equal to the average wind speed in the roadway is accurately determined by the post-processing function. This provides a basis for accurately determining the air volume of large-size roadway. Finally, in order to facilitate the use, the calculation model is packaged as an APP that can run independently. Only by inputting the geometric parameters of the roadway and the inlet wind speed, the position of the average wind speed contour in the tunnel can be determined.