Abstract: In order to improve the stability margin of the fan, the stability improvement mechanism of the fan by applying the single circumferential groove treatment to a FBCDZ-No20 contra-rotating fan is numerically investigated, and the influence of the axial position of the circumferential groove on the aerodynamic performance and stability improvement ability of the fan was explored. The results show that the single circumferential groove casing treatment of the front impeller can improve the stability margin of the fan, and the axial position of the single circumferential groove has an impact on the aerodynamic performance and stability improvement of the fan. When the single circumferential groove is located at the leading edge of the front blade and 25% of the axial chord length of the blade tip, considerable stability margin improvement can be achieved with a higher peak efficiency loss. When the single circumferential groove is located at 50%, 75% of the axial chord length of the blade tip and the trailing edge, respectively, the stability improvement effect gradually weakens, and the corresponding efficiency loss also gradually decreases. The circumferential groove increases the axial momentum of the leakage flow at the blade tip and increasing the relative airflow angle at the inlet of front impeller through the extraction of the low-energy fluid from the front edge of the blade pressure side, transporting it along the circumferential direction to and ejecting at the leading edge of the blade suction side, changing the trajectory of the leakage flow at the blade tip, suppressing leading edge overflow and boundary layer separation at the trailing edge, thus reducing blockage in the blade channel and delaying stall inception, achieving stability improvement. Neither the stage where the stall first occurred in the contra-rotating fan nor the stall inception type is changed after the application of the single circumferential groove casing treatment.