Abstract: Because LPG generally is a multi-component mixture of propane, propylene, butane and butylene etc, the mass ratio of each component for gas phase or liquid phase varies during the spontaneous vaporization. The mass or volume fraction of each component can be determined by the empirical formula or experimental data only for two component LPG mixture of propane and butane, the variation of other LPG composed with multi-component must be determined by chromatographic analysis. In this paper a model is established to simulate the LPG component mass ratio change during the spontaneous vaporization according to the theory of chemical phase equilibrium, principle of mass conservation and equation of state of real gas. Because the model is differential and algebra equations set, a difference equation set is presented. Based on the solution the curves are given for the mass fraction variation of each component of any multi-component LPG mixture in the spontaneous vaporization process and can be used to actual engineering. At the same time the analyses are made on the changes of saturation pressure, Warburg's number and combustion potential of LPG caused by the mass ratio change of each component during the spontaneous vaporization.