Abstract: Based on the measured data of flood peak discharge in July 2018, the section-scale bank collapse model (BSTEM) was used to simulate the neck cutoff process and cutoff time under the three different erosion modes (upstream, downstream, combined upstream and downstream) for reproducing the neck cutoff event triggered by bank erosion in the lower reaches of the Black River of the Yellow River Source.The results of the combined upstream and downstream erosion mode show that the time of the flow intersection at the neck is on July 15 2018, which is consistent with the time of the actual bank collapse triggering the neck cutoff.The cutoff time of the combined erosion mode is 2 days or 6 days earlier than that of the upstream or downstream bank erosion mode, respectively.The accelerated bank erosion caused by this extreme flood event is the direct driving force of neck cutoffs occurred in advance in the lower Black River.