Combines theories of gas dynamics, fluid dynamics and numerical heat transfer theory, the condensing flow characteristics of water vapor in wet natural gas within the Laval nozzle were studied. A mathematical model was developed to predict the spontaneous condensing phenomenon in the supersonic flows using the classical nucleation and droplet growth theories. The numerical approach is validated with the experimental data by using UDF and UDS modules in FLUENT software, which shows a good agreement between them, and showed that the mathematical model can better predict the parameter changes in the condensation process. The condensation characteristics of water vapor in the Laval nozzle are described in detail. The results show that the condensation process was a rapid variation of the vapor-liquid phase change both in the space and in time, the distribution of nucleation rate is restricted to a small area. The spontaneous condensation of water vapor will not appear immediately when the steam reaches the saturation state. Instead, it occurs downstream the nozzle throat, where the steam is in the state of supersaturation. The previous accumulation of Supersaturation has led to a nuclear process occurring in a very short time. The degree of supercooling was also dramatically reduced in this small area, and when it is below the supersaturation limitation, the nucleation process ceases to occur.