Air-hydrocarbon mixture gas is a combustible gas made by mixing the vaporized liquid light hydrocarbon and air with a specific ratio, which is a kind of clean fuel. However, the adopted light hydrocarbons are liquid state at standard condition, causing the high dew point of the mixing gas. In this work, the nucleation thermodynamic theory was used to study the phase transition or nucleation force caused by the changes of temperature and pressure on some zones, and the nucleation energy of droplets. The nucleation mechanism, nucleation energy and their relationships with the subcooled temperature and supersaturation of the air-hydrocarbon mixture gas were obtained from these studies. It was shown that the subcooled temperature and supersaturation required by phase transition or nucleation decreased with the increase of temperature and pressure. Although the subcooled temperature and supersaturation required for complete phase transition would unlikely be reached due to the fluctuation of the working condition, the subcooled temperature and supersaturation required for nucleation can easily take place. Therefore, it is important to control the occurrence from nucleation to complete phase transition for preventing the formation of dew point. The research results provided the reference for the research on the safety of storage and transportation of air-hydrocarbon mixture gas.