The invention discloses a drainage method based on a siphonic roofing drainage structure. The siphonic roofing drainage structure comprises gutter spout funnels and downspouts. The drainage method comprises the steps that first, the siphonic roofing drainage structure is constructed; second, data of the water amount of roofing accumulated water are collected; third, whether data collected by a pressure sensor within the first specified time are less than a preset pressure threshold value or not is judged; fourth, small-flow accumulated water is drained; and fifth, large-flow accumulated wateris drained. By arranging annular reverse ridges on a roofing, flow-guiding pipes with solenoid valves are arranged on the lower parts of the annular reverse ridges, the solenoid valves can be circularly opened and closed according to the actual situations, drainage modes corresponding to the roofing accumulated water amount are selected according to the different roofing accumulated water amount,thus the roofing drainage efficiency can be greatly improved, by arranging pressure sensors in the flow-guiding pipes and arranging liquid level sensors on the outer sides of the annular reverse ridges, roofing drainage can be automatically controlled, the labor intensity of workers can be effectively reduced, and the safety of the workers is ensured.