Abstract: In the literatures available, three basic methods are found for the theoretical calculalion of explosion limit: ①expirical fomula for calculating the explosion limit of a flammable gas; ②fomula for calculating the explosion limit of a mixture of multi-flammable gases; ③the method for calculating the explosion limit of the mixture of flammable gas and inert gas. The gas in oil storage tank is a mixture of oxygen, inert gas and several other flammable gasses. Due to the different compositions and fractions of crude oils, it is difficult to determine universally the explosion limit of this gas mixture, and the explosion limit determined can only be suitable for the specific oil in some oil field. A theoretical calculation method is concluded in this paper based on the actual, applications: separating off the air from the gas mixture, considering the remaining mixture as one unit and calculating the explosion limit of the mixture of oxygen, int ert gas and other flammable gases based on corresponding formula.