The residence time distribution (RTD) of centrifugal pump was studied with a pulse tracer method. The back-mixing characteristics were described qualitatively by comparing the RTD of centrifugal pump with that of blank pipe. Three flow control modes, pre-pump valve, outlet valve, and motor speed, were selected to examine their effects on RTD. The experimental results indicate that the RTD curve of centrifugal pump shows a single-peak distribution, and dead zone is found. The back-mixing of centrifugal pump is greater than that of blank pipe with equal diameter and volume. The dimensionless variance ranges from 0.39 to 0.61, which is not close to ideal plug flow or complete mixing flow. The back-mixing is significantly affected by flow rate, it declines apparently with the increase of flow rate. Different flow control modes have a significant impact on the back-mixing. The back-mixing of centrifugal pump controlled by motor speed is greater than that regulated by valve, and pre-pump valve mode is similar to outlet valve mode. As the flow rate decreases, the difference of RTD among three control modes becomes more significant. The maximum relative deviation reaches 13.6%.