The performance of centrifugal pumps during transient operating periods has been obtained increasing concern recently. In this paper, the transient behavior of a prototype centrifugal pump during rapid startup period is studied experimentally and theoretically. The variations of the rotational speed, flow rate, and head during the startup period are recorded in the experiment. The non-dimensional flow rate and head are used to analyze the transient behavior. Then, the additional theoretical head during the startup is calculated quantitatively by means of the generalized equation of turbomachinery. The research result shows that there exists clear transient effect during the rapid startup period, and the quasi-steady analysis is unable to accurately assess the transient flow.