Interference mitigation becomes increasingly important in Wireless Body Area Networks (WBANs) when multiple co-located WBANs operate on the same or adjacent channels. In this paper, we propose an efficient decentralized interference mitigation (DIM) scheme for coexisting WBANs. The superframe is divided into Beacon, scheduling phase (SP) and contention access phase (CAP), in which DIM uses CSMA/CA in CAP and scheduling in SP. In DIM, hub will adjust the length between SP and CAP dynamically according to its recent network performance. Specifically, the length of SP will be reduced when the channel utilization in SP decreases, and will be expanded on the contrary. DIM combines the SINR and interference performance. Nodes in the overlapping region tends to be interfered and will get lower access priority. Hub allocates the slots to the nodes based on the DIM algorithm. The simulation results show that the DIM algorithm archive much higher throughput and lower latency under the interference scenario.