The overflow port is an important part of the runoff control facility, but the hydraulic characteristics of the overflow port are currently in shortage of specific research. The design formula of the overflow port in the stormwater management manual at home and abroad was incomplete, which was not sufficient to guide the design calculation. As a representative of the overflow port, the thin-annular weir could also be considered as a macroscopic expression of the 'confluence’. Its height and inner diameter were two geometric elements that characterize the flow-head relationship. The flow-head relationship in different heights and inner diameters was measured by laboratory tests. The results showed that the height of the weir had a certain influence on the overcurrent capability of the thin-annular weir. The dimensionless curve was drawn for the annular thin-walled concrete with different inner diameters, and the general formula was obtained. This research could provide reference for the design calculation of the overflow port in sponge city facilities.