The embodiment of the invention relates to the technical field of rainwater collection and purification of sponge cities, and provides an overflow opening structure of a biological detention facility.The overflow opening structure comprises a low-return-period overflow pipeline, an overflow well and a siphonage pipeline, a pipe opening of the low-return-period overflow pipeline is lower than a wellhead of the overflow well, one end of the siphonage pipeline is connected with the low-return-period overflow pipeline, the other end of the siphonage pipeline is connected with the overflow well, the end, connected with the low-return-period overflow pipeline, the siphonage pipeline is higher than the end, connected with the overflow well, of the siphonage pipeline, and the lower part of the overflow well communicates with a rainwater pipeline. The overflow opening structure of the biological detention facility has multi-stage overflow to achieve overflow drainage at different return periods, a siphonage drainage mode is adopted to increase the instantaneous drainage flow rate and improve the overflow capacity, the problems of insufficient overflow capacity and facility water accumulating of a biological detention facility are avoided, the overall structure is simple, the cost is low, the effect is significant, and use mounting and later operation and maintenance are convenient.