The invention discloses a device for starting a side slope siphoning drainage system and a method of the device. The device comprises a water tank, air discharge pipes and a drainage pipe, a water outlet of each siphon is formed in the sealed water tank, and an inlet of each air discharge pipe is connected to the top of the water tank; an inlet of the drainage pipe is connected to the bottom of the water tank, the vertical distance between the inlet of the drainage pipe and an outlet of the drainage pipe is larger than the height of a water column corresponding to the local atmospheric pressure, and the height difference between the top of the water tank and a hole opening of a drilling hole is larger than the height of the water column corresponding to the local atmospheric pressure. According to the device, by controlling opening and closing of the draining pipe, negative pressure is produced to start siphoning without the help of other equipment, and the device is simple and reliable; the vertical distance between the inlet of each air discharge pipe and an outlet of the corresponding air discharge pipe is larger than the height of the water column corresponding to the local atmospheric pressure, negative pressure is formed in the water tank, water in the air discharge pipes cannot enter the water tank, it is guaranteed that negative pressure in the water tank is not broken,and accordingly the stability of the whole device is improved; multiple siphons can be started simultaneously, and siphoning is started quickly and efficiently.