Based on the characters of high-pressure, high-speed and small-size of centrifugal fans used in helicopters, the basal structure parameters of the fans were analyzed and calculated. Efforts were made to analyze the flowing rules of the gas in the impeller. Influence of distribution of the gas vortex on the across-section was discussed. Restriction requirements of selecting the gas vortex was put forward. The shape of the meridian-section and the across-section was designed. It was proved that the efficiency of the centrifugal fans can be evidently improved by reasonably determining the distribution of the gas vortex.