The invention relates to a building roof siphon type rainwater drainage system capable of achieving energy recovery. Currently, a building roof siphon type rainwater drainage system which is reasonable in structural design and capable of achieving energy recovery is not available. The building roof siphon type rainwater drainage system capable of achieving energy recovery is characterized by comprising a siphonic roof drain located in a center gutter, a hanging frame fixed to a floor slab below the center gutter, a rainwater connecting pipe, a hanger beam, a rainwater hanging pipe, hanging pipe clamps used for fixing the rainwater hanging pipe, a vertical rainwater pipe, horizontal pipe clamps, an output wire, a storage battery, an indoor rainwater inspection well and a hydro generator installed inside the indoor rainwater inspection well; the upper end of the rainwater connecting pipe is connected with the siphonic roof drain, and the lower end of the rainwater connecting pipe is connected with the hanging pipe clamps; the horizontal pipe clamps are fixed to the vertical rainwater pipe; the upper end of the vertical rainwater connecting pipe is connected with the rainwater hanging pipe; the lower end of the vertical rainwater pipe is located inside the indoor rainwater inspection well and matched with the hydro generator; the storage battery is connected with the hydro generator. According to the building roof siphon type rainwater drainage system capable of achieving energy recovery, the mechanical energy of rainwater is converted into electric energy, and the energy recovery efficiency is high.