The invention discloses an automatic starting type side slope siphon drainage system and method with the flow rate monitoring function. The system comprises a drilled hole, a siphon, a water tank, a clamping plate, an exhaust pipe, a drainage pipe and a float valve, wherein the water outlet of the siphon is arranged in the sealed water tank; the inlet of the exhaust pipe is connected with the sidewall of the water tank; the inlet of the drainage pipe is connected with the bottom of the water tank; sliding blocks are arranged on the periphery of the float valve; a sliding chute is formed in the clamping plate; the clamping plate controls the float valve not to have transverse movement; the perpendicular distance from the inlet of the exhaust pipe to the outlet of the exhaust pipe is greater than the water column height corresponding to the local barometric pressure; and the height difference between the water tank and the opening of the drilled hole is greater than the water column height corresponding to the local barometric pressure. During the using process, the automatic starting type side slope siphon drainage system frequently carries out fast and efficient siphon starting, solves the problem about the siphon starting efficiency, also solves the problem that during the draining and using process of the siphon, the siphon needs to be regularly restarted due to air accumulation in the siphon, meets the demand that the side slope effectively siphons and drains water and can monitor the flow rate of drainage of the siphon.