Analysis of the transient hydraulic characteristics of a centrifugal pump during its speed regulating process was carried out based on the approaches of both quasi-steady-state and transient numerical simulation with the CEL languages function applied by using the commercial software ANSYS CFX 12.0. Meanwhile, the effect of different impeller acceleration and regulating flow on the transient pump head was also studied in the paper. To avoid the influence of the artificial factor in the simulations, a software Flowmaster was used for boundary condition definitions. The numerical results show that pump head deviation between the transient state and quasi-steady-state becomes large with the impeller acceleration increasing and the regulating flow rate decreasing. The influence of impeller and flow acceleration in speed regulating period can be accurately predicted by transient simulation method based on CEL languages function. Additionally, by investigating flow section in the centrifugal pump, the transient flow in its speed regulating process was obtained, which can be used to judge the speed regulating schemes from the inner flow pattern, and find the dynamic root for induced unsteady total pressure and its generated positions, as well as provide certain reference for the centrifugal pump hydraulic design.