Abstract: The leakage volume of natural gas is an important factor affecting the consequences of pipeline leakage and explosion. Restricted by the ratio of methane and air concentration, not all of the leakage gas will explode. Therefore, a calculation model for the explosive peak overpressure of premixed methane-air gas was established under unconstraint conditions. The relationship between the volume of premixed methane-air gas and the peak overpressure was simulated and analyzed with the Fluent software, and the calculation model was verified. Moreover, the law between the effective explosive volume of methane and the peak overpressure was obtained, and a method for calculating the effective volume was proposed. The results show that the propagation time of the explosion peak overpressure obtained by the numerical simulation model is consistent with the experimental results, and the attenuation law of the explosion peak overpressure is consistent with that as indicated by the existing research. Additionally, the effective volume of methane involved in the explosion is about 0.11 to 0.12 times the actual volume of the premixed gas, and the precision is high in the case the actual premixed gas volume is large. Hence, the results can provide a theoretical basis for the simulation and prediction of natural gas explosion, the consequence evaluation and the configuration of protection resources.