With the expansion of the engineering application of solid-liquid two-phase flow, the overload characteristics of low-specific-speed sewage pumps become an important obstacle to the development of the products. In this paper, the traditional diffusion-type impeller is utilized to carry out hydraulic design of a low-specific-speed sewage pump. And on this basis, the thickness of the blades is increased for getting an impeller with contractive-type flow channel. It was found out that through external characteristics, the efficiency of contraction-type impeller is slightly lower than that of diffusion-type impeller, but it shows obvious non-overload performance. PIV experiment is arranged to find the inherent. The results show that although internal flow field distribution of contraction-type impeller is better than that of diffusion-type impeller, the severe uneven distribution of absolute velocity of the impeller outlet along the circumference is one of main reasons of inefficiency. In addition, another five impellers with different contraction ratio are designed to research relationship between contraction ratio and character of non-overload. The characteristics are predicted by CFD, and after comparing the results, it is found out that, the smaller the contraction ratio is, the closer the condition of power extreme is to the design condition, which has important implications for the engineer.