In order to efficiently utilize the ultra-low head water resources, an axial flow turbine with a siphon outlet conduit was designed, and three-dimensional CFD numerical simulations were conducted with the design head and rated runner speed. The hydraulic loss of each flow passage component was calculated to study the hydraulic performance of the siphon turbine. The relationship between the flow regime at the runner outlet and the head loss of the outlet conduit was analyzed by changing the runner blade airfoil, and the influence of different runner blades on the hydraulic performance of the siphon turbine was studied. The results show that, with a constant water head, runner speed, and guide vane opening, the head loss in the outlet conduit of the turbine in the modified scheme with the third blade is at a minimal value of 0. 135 m, the corresponding average swirl angle is 13. 26o, and the turbine efficiency is at the maximum(89. 33%). In addition, the influence of the number of blades on the hydraulic performance of the siphon turbine was analyzed through selection of the blade with the maximum efficiency.