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NbOx-based devices exhibit intriguing promise for beyond-CMOS applications due to their dynamic threshold switching (TS) and negative differential resistance (NDR) behaviors. However, an in-depth study on the degradation scheme of such a device is absent. In this work, we investigate the degradation behavior, i.e., the shift of switching voltages ( ${V}_{\text {th}}$ , ${V}_{\text {hold}}$ ) and the shrink of voltage window (VW), of a nanoscale forming-free TiN/NbOx/TiN memristor. Through electrical tests and random telegraph noise (RTN)-based defect tracking, we proved that the shrink of the VW and the increase of switching voltages originate from the increase of electrode resistance due to the oxygen vacancy accumulation. According to the elucidated degradation mechanisms, we propose a reverse refresh strategy to extend the endurance and delay VW degradation. This work provides a possible view of NbOx devices’ degradation and may promote the applications.