Siphon drainage system for negative pressure method or drainage system, which is widely used in large-scale roof buildings, such as stadiums, factories and other large span roof structure complex, is an effective way to solve the roof drainage. Siphon roof drainage system consists of anti-vortex water bucket, rain suspension, Storm riser, buried pipes, rainwater from households tube. Siphon drainage siphon, 5 minutes wave phase flow, pulse stream (pulsating flow), plug flow (drawing flow), foam flow (emulsion flow), full pipe flow, etc. In the early rainfall, rainwater drainage system suspended rainwater pipe is non-full pipe flow to flow and pulse wave flow-based, the system is gravity flow state. As rainfall increases the depth of the bucket before gradually increasing water flow and a gradual transition to the foam flow piston and intermittently siphon full pipe flow pattern, the more obvious negative occur within the suspension tube. Formation siphon drainage capacity of the system suddenly increases, the depth will come down before the fight, the system back to the way gravity flow. This transformation will continue for some time back and forth until further increased rainfall, the former tend to be stable bucket depth, mixed gas system to reduce and eventually form a stable siphon full pipe flow. According to specification, suspended gravity flow pipe roof rainwater drainage system shall be non-full flow design, its fullness should not be greater than 0.8, the flow rate of the tube should not be less than 0.75 m / s, and a slope of not less than 0.5%, a larger overhang Hanging tube diameter and slope. Meanwhile, in order to make each rain water bucket on a rain water pipe with the normal discharge, thus limiting the amount of a water bucket rain suspended pipe shall be more than four, which also led to the suspension of rainwater and stormwater pipe riser number increases, while increasing the roof loads, but also increases the cost of the project. Siphon roof drainage system to improve the drainage capacity, can quickly rule out the roof rainwater, reducing roof loads, reducing roof water damage to the building roof structure; the number of tubes hanging water bucket unrestricted access, saving the rain riser; suspension tubes do not need to slope, easy installation and beautiful, in the mechanical and electrical installation space is always just keep an installation height for easy electromechanical integrated and coordinated; no need to set checkpoints or clean mouth (because siphon water bucket design can be separated cut trash and large particulate matter into the system and the system has a self-cleaning function); the system pressure by siphon flow calculations can choose to reduce the diameter of the pipe, the project proved to significantly reduce the project cost.