A controllable siphon drainage device of reservoir drainage and flood discharging of the invention comprises a dam body, a siphon pipe, a gas outlet valve, a gas outlet pipe, a water inlet pipe, a water inlet valve, a gas inlet pipe provided with a valve, a gas-liquid separator, a water charging pipe provided with a valve and a purge valve. The invention is characterized in that: the dam body is provided with a siphon pipe, the entrance end of the siphon is arranged in the lower part inside the reservoir dam body, the exit end of the siphon is provided with an elbow arranged in the lower part outside the reservoir dam, the middle part of the siphon is arranged on the top of the dam body, the siphon pipe is provided with a gas inlet pipe with a valve, the siphon pipe is connected to the gas-liquid separator through the gas outlet pipe and the water inlet pipe, the gas outlet pipe is provided with a gas outlet valve, the water inlet pipe is provided with a water inlet valve, the top end of the gas-liquid separator is provided with a purge valve and a water charging pipe with a valve. The invention has the advantages of simple operations, convenience and quickness, work-saving and labor-saving; can improve the effective height, reservoir capacity, flood charging and discharging and normal drainage ability, thereby improving the reservoir safety and flexibility.