As shown in Fig. 4, the volume distribution of vapour on midspan of impellers is mostly nearby the suction side of blade in the inlet, and extends along the streamline direction with the increase of blade number. The more blade number in the impeller, the overflowing area of flow passage decreases, the more crowding cut effect phenomenon occurrences, flow passage was clogged evidently. So the cavitation characteristics of the model pump became worse, which is concordant with the theoretical analysis.