Abstract: The rotating fan is prone to instability such as stall and even surge when operating at low flow rates. In order to study the influence of axial spacings on the stall inception and its development and propagation process of a contra-rotating fan , a Shear Stress Transport(SST)k-ωturbulence model was used to numerically simulate the unsteady flow in the whole passage of a FBCDZ-10-No20 contra-rotating fan. The stall evolution of fan under two axial spacings are investigated. The results show that the axial spacing has significant influence on the inception and development of fan stall.In the stall inception stage, when the axial spacing between the two-stage impellers is 170 mm, the stall inception first occurs within the rear impeller, which locates in the trailing edge area on the suction surface of the blade root. Then stall inception develops from the blade root to the tip area along the radial direction, and accumulates towards a blade passage along the circumferential direction and falls off on the way to form a blocking area, which eventually leads to stall. When the axial spacing is 70 mm, the stall inception successively occurs the tip area of the two-stage impellers. Then stall inception keeps increasing, which eventually leads to stall. During the full stall stage, the axial spacing has a significant effect on the propagation of the stall vortex in the circumferential, axial and radial directions. When the axial spacing is 170 mm, the type of stall vortex shows the single vortex full-blade high stall. The propagation range of the stall vortex in the axial direction is limited to the region of rear impeller, and rotates at 33.3% of the rear impeller speed in the circumferential direction; When the axial spacing is 70 mm, the type of stall vortex shows the multi-vortex partial blade high stall. The stall vortex are successively generated in the tip area of the two-stage impellers, propagating upward and downstream in the axial direction, and are dispersed in the area above 70% of the blade height of each blade channel in the radial direction. Due to the change of the axial distance between the two-stage impellers, the type of stall inception of the fan is changed from “partial surge type” to “Spike-type”.