Both the impeller and inlet guided vanes were studied. Considering the inlet prewhirl by using radial inlet guided vanes, the calculation formula of the pump’s maximum shaft power and its corresponding flow rate were deduced. The effects of inlet guided vanes’ geometric parameters on theoretical head curves, power curves, theoretical maximum flow rate, maximum shaft power and its corresponding flow rate were also analyzed. Then eight schemes with different geometric parameters were numerically simulated by using CFD software, the validity of the formula was verified by comparing numerical result and formula calculation, indicating that it can provide guidance for the design of non-overload centrifugal pump.