To study the effect of the supersonic nozzle structure on the spontaneous condensation of water vapor, the wet steam model used to study the wet steam liquefaction of steam turbine was introduced into the numerical study of supersonic nozzle in supersonic cyclone separator, and the numerical results was in good agreement with the experimental data of the related literature. The supersonic nozzles, the contraction section with the Witozinsky curve and the bicubic curve, and the expansion section with conical curve and Foelsch curves, were designed. The influence of supersonic nozzle structure on water vapour liquefaction ability was investigated. And the following research results were obtained. When the structure of the supersonic nozzle was the same, the Witozinsky curve was selected for contraction section of the supersonic nozzle, and the nucleation rate of the droplet in the nozzle increased by 21.97 % and the number of droplets increased by 32.16 %, which was helpful to the gas liquefaction in the nozzle. When the contraction curve of the supersonic nozzle was consistent, and the expansion curve was Foelsch curve, the gas nucleation rate and the number of droplets in the nozzle were greatly improved. The distribution of nozzle humidity was related to the structure of expansion section, but not to the structure of contraction section. The maximum humidity in nozzle depended on the outlet diameter of nozzle. When the other structures of the nozzle were consistent, the cone angle of divergent section increased from 2° to 8°, the peak value of the nucleation rate increased by 493.15 %, the maximum of the droplet number increased by 246.12 %, the outlet humidity of the nozzle increased by 73.37 %, which greatly improved the gas liquefaction rate. In the case of gas liquefaction, increasing the cone angle of expansion section was conducive to liquefaction of water vapor.