The non-equilibrium condensation of water vapor in moist air significantly affects the structure of the external flow field around the RAE 2822 airfoil and then changes its aerodynamics. During transonic flow, there are various factors closely related to an airfoil’s aerodynamic properties and its performance, such as air humidity and the attack angle of the oncoming flow. Considering the significant role of water vapor in moist air in transonic or supersonic flow, a detailed discussion on the flow mechanism of non-equilibrium condensation at supersonic speeds is necessary. First, a mixed condensation model, considering the influence of droplet growth over the entire range of size which the Knudsen number model contains, was established to provide a clearer research and analysis of the characteristics and mechanism of condensation flow. Furthermore, a mixed model was adopted to investigate the impact of relative humidity on the surface pressure coefficient, lift, and drag of the wing. Finally, calculations were conducted to probe into the influence of different angles of attack on the aerodynamic properties of the RAE 2822 airfoil. The results for calculations indicate that non-equilibrium and homogeneous condensation of water vapor in moist air occurs only in some areas on the top surface of the airfoil in transonic flow, and the lift and drag coefficients of the wing vary under different relative humidities. Generally, with an increase in relative humidity, both the lift and drag coefficients increase.