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In order to analyze the flow influence mechanism of axial-flow pump on the siphon outlet passage, the three-dimensional internal flow of an axial-flow pumping system with siphon outlet passage was simulated based on CFD method under different running conditions. The relationship between guide vane outlet velocity circulation and flow rate had obvious influence on hydraulic loss and internal flow field of siphon outlet passage. The flow field characteristics of siphon outlet passage were analyzed qualitatively, and the mathematical relationship between them was analyzed quantitatively. The research results indicated that there was great difference between internal flow field of siphon outlet passage under different running conditions, and the hydraulic loss of siphon outlet passage was mainly concentrated on flow passage before hump section. The average value of velocity-weighted average swirl angle was 52.34°, and it had small fluctuation in the flow coefficient range of 0.35~0.70 for hump section. With the increase of flow coefficient, the axial velocity distribution uniformity of hump section increased gradually, while the guide vane outlet velocity circulation decreased firstly and then increased, and the minimum velocity circulation was in the high efficiency area. Guide vane outlet velocity circulation made the relationship of hydraulic loss of siphon outlet passage and flow rate more complicated, and there was no quadratic regression relationship between hydraulic loss and flow rate. These study results were available reference for design optimization of siphon outlet passage.