The amount of initial gas release after the destruction of coal by ground stress relates closely to the outburst danger. In this paper, the following method was proposed to accurately determine the initial gas desorption law of coal. First, real-time temperature and pressure data of sealed coal sample tank after the sudden exposure of coal that had reached adsorption equilibrium within the tank were collected first via a high-precision, high frequency response sensor. Then, the mass flow of gas passing through the pneumatic component at different times was calculated according to flow rate characteristics of pneumatic component that communicated with the atmosphere on the tank. In this process, it is determined that a convergent nozzle with a larger contraction ratio should be used as the channel for gas desorption and release. Moreover, in light of the characteristics of the testing process, the existing parameters that characterize the flow characteristics of pneumatic component were optimized and measured. Finally, the initial gas desorption laws of two coal samples with different metamorphic degrees were measured and analyzed under two pressures using three different-sized convergent nozzles.