Centrifugal pumps always work under steady conditions,and many researches focus on the steady operation.But transient conditions,such as sudden startup and shutdown,are inevitable.The researches on the inner flow of centrifugal pumps under transient conditions have been done,and they show that the transient operation is different from the steady operation.In order to research the evolution of unsteady flow in a centrifugal pump under transient conditions,and to investigate the mechanism of transient effects by analyzing the unsteady flow in a centrifugal pump,the external characteristic experiment and the internal flow numerical calculation of the centrifugal pump with an open impeller during startup is presented.The relationships of the rotation speed,capacity and head between start-time are obtained by the external characteristics experiment.The numerical calculations under startup process are carried out by using the k-e model and N-S equation.The distribution of velocity and pressure in the inner channel of the tested pump was obtained by choosing fourteen start-time points and twelve geometrical points in the impeller channel during startup.The calculation results show that the velocity and the pressure increase linearly with the start-time before rotation’s speed gets steady,then changes almost horizontally after rotation speed becomes steady,then fluctuates until being steady.The internal flow characteristics are in good agreement with the external characteristic experimental results and numerical calculation.The simulation methods and results make the basis for the diagnosis and optimization of under flow in the centrifugal pump during transient operation. Centrifugal pumps always work under steady conditions, and many researches focus on the steady operation.But transient conditions, such as sudden startup and shutdown, are inevitable. The researches on the inner flow of centrifugal pumps under transient conditions have been done, and they show that the transient operation is different from the steady operation. order to research the evolution of unsteady flow in a centrifugal pump under transient conditions, and to investigate the mechanism of transient effects by analyzing the unsteady flow in a centrifugal pump, the external characteristic experiment and the internal flow numerical calculation of the centrifugal pump with an open impeller during startup is presented. The relationships of the rotation speed, capacity and head between start-time are obtained by the external characteristics experiment. Numerical calculations under startup process are carried out by using the ke model and NS equation. The distribution of velocity and pressu re in the inner channel of the tested pump was obtained by choosing fourteen start-time points and twelve geometrical points in the impeller channel during startup. Calculation results show that the velocity and the pressure increase linearly with the start-time before rotation’s speed gets steady, then changes almost horizontally after the rotation speed is steady, then fluctuates until steady, then the internal flow characteristics are in good agreement with the external characteristic experimental results and numerical calculation.The simulation methods and results make the basis for the diagnosis and optimization of under flow in the centrifugal pump during transient operation.