A mathematical model was developed to investigate the two-component mixture spontaneous condensation. Numerical simulations were performed for the two-component mixture flow in Laval nozzle. The comparison between numerical and the experimental results shows that the model is accurate. The influences of the expansion rate of the nozzle on the spontaneous condensation and flow characteristics were investigated. It was found that under the same conditions, as the expansion rate increases, the Wilson point moves to the inlet side, the maximum supercooling and the nucleation rate and the droplet number also increase, the mean radius of the liquid droplets decreases and the vapor in the mixture reaches its supersaturated state earlier.