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The efficiency of axial-flow pump decreases rapidly on both sides of the optimal flow condition, there is a saddle pattern unstable flow range at low flow side. It is of practical significance to study the internal flow field and external characteristics of axial-flow pump under full flow range, and put forward improvement methods to broaden the workable flow range. The relative flow angles at different radial positions varied along the impeller trailing edge. When the flow decreased, the flow surface component of absolute velocity at different radius decreased at the same step, but the circumferential component of absolute velocity varied greatly according to the relative flow angle of their respective velocity triangle, resulting in the difference of head performance at different radii, the flow deflected towards the outer edge of the passage, and a backflow and vortex area at hub side after impeller was formed. This axial-flow model featured a built-in motor inside the flow passage, so the flow out of the guide vane was a diffusion flow. When the flow separation conditions were met, the boundary layer separation would occur on the inner wall of the pipe. The vortex would develop from the boundary layer to affect the movement of the mainstream. Generally, the methods to control flow separation included: controlling the deceleration mode of the mainstream, eliminating the boundary layer by bypass the boundary flow or injection a flow, turning the laminar boundary layer into a turbulent boundary layer, and separating the flow passage and so on. For this model, the approach of adding a separator to form the double-layer flow passage can deal both the difference of the circumferential component of absolute velocity at different radii after impeller and reduce the downstream flow separation. The geometry of this structure was characterized by the addition of an inner conical tube at hub side. The tube started closely from the impeller trailing edge and extended into the guide vane. The ratio of the distance between the tube wall and the guide vane hub to the guide vane channel width was denoted as geometric variablesλ, the effects of the differentλvalues on the external characteristics and internal flow field of the axial flow pump were studied. The results showed that the double-layer flow passage after the impeller can increase the head value when running inside the unstable flow range, reduce the positive slope value of the flow-head curve, and expand the safe operation flow range. When the flow was greater than the design flow, the flow separation after the guide vane was reduced and the head value was significantly increased. By comparing and analyzing the simulation results under three λ values at 1/3, 1/4 and 1/5 respectively, it was found that the head improving effect was the best when λwas 1/3, and the difference between the three λcases was less than 4%.