The siphon formation process with different section forms was simulated by using the VOF model considering gas compressibility, which was verified by an experiment. The effect of section forms on the hydraulic characteristics of the siphon outlet was analyzed from the three aspects of velocity distribution, hydraulic loss and siphon formation time. The results show that the results between simulation and experiment are in good agreement. The section forms have great influence on the hydraulic characteristics of the siphon outlet passage. Rationally designed gradual change section can uniformly distribute the velocity and pressure distribution inside the passage, but it can increase the head loss of the system. Circular pipes have the least hydraulic loss, but the exhaust capacity is weaker than that of square pipes.