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In the process industry, there are many cases where the residual energy liquid has a high-pressure head and low flow rate, it is necessary to use the multistage guide vane centrifugal pump as turbine (PAT) to recover the liquid residual energy. To adapt to the impact of production regulation on the performance of pumps as turbines in the process industry, multistage PAT is required to have a small variation in output power with the flow, i.e., a flatter output power-flow curve. Based on the Euler equation and according to the retention of the velocity distance, the relationship between the output power and geometrical parameters of the guide vane PAT was derived, the output power was related to the geometrical parameters of the guide vane (throat area, outlet angle, guide vane number, and base circle diameter) and the geometrical parameters of the impeller (blade outlet diameter, outlet width, blade outlet angle, and blade number). Using mathematical derivatives, the influence of geometric parameters of the guide vane on the flatness of the output power curve was determined. A two-stage PAT was used as the object research, and the research scheme was designed by changing the geometric parameters of the positive guide vane, which were experimented with and simulated by Fluent software. The computational results were consistent with the theoretical derivation, the output power curve can be flattened by appropriately increasing the positive guide vane throat area, positive guide vane outlet angle, positive guide vane blades number, or appropriately reducing the positive guide vane base circle diameter; at the design condition point, the order of influence of positive guide vane geometric parameters on the slope of the power curve was throat area, outlet placement angle, blade number, base circle diameter, and the slope of the power curve was reduced by 0.17, 0.11, 0.05 and 0.03, respectively; a proper increase in throat area can lead to an increase in multistage PAT efficiency by 1.65 percentage points and a shift in the high-efficiency point toward high flow rates.