Abstract: The use of small circulating pump can make gas water heater realize zero waiting for hot water. In view of the special operating environment of this type of pumps, a volute profile cutting design method was proposed. Taking a centrifugal pump for gas water heater as the research object, the value of volute cutting θ_s was changed, and the numerical simulation method was adopted to investigate the impact of different circumferential cutting positions on hydraulic performance and flow field of the pump. The results show that the cutting volute profile makes flow cross-sectional area of the volute to firstly decrease and then increase in the corresponding circumferential range, thus a low pressure area is formed on the cutting surface of the volute. However, when θ_s=150°, the low-pressure area happens to be at the outlet end of the blade, and more fluid medium flows into the volute through the suction side of the blade. The side pressure difference increases and strengthens, so that the head of the centrifugal pump increases by about 2.7% under the Q=7 L/min working condition on the basis of maintaining the large flow condition. However, cutting volute destroys the curvature continuity of the volute profile, which results in flow deterioration and low efficiency under various operating conditions.