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In order to adapt to the influence of production regulation on turbine performance in chemical process industry, it is required that the head of pump as turbine changes little with the flow rate, which makes the head-flow curve relatively flat. On the premise of conservation of velocity moment in volute of pump as turbine, the relationship between the theoretical head, the flow rate and the geometric parameters of the turbine was derived. The four main geometric parameters affecting the slope of the headflow curve were obtained by studying the relationship: wrap angle of volute, blade outlet angle, blade outlet edge and blade number. A low specific speed pump was selected as the research object for pump as turbine. Thirteen research schemes were determined by changing the geometric parameters of pump as turbine. The wrap angle of volute, blade outlet angle, blade outlet edge and blade number were changed in thirteen research schemes and simulated and tested by ANSYS-Fluent software. The results showed that the head-flow curve became flat with the decrease of wrap angle of volute and the increase of blade outlet angle within a certain range, and the optimal blade outlet position and blade number made the head-flow curve more flat. The influence of wrap angle of volute on the slope of head flow curve was remarkable.