Reduced-pressure type backflow preventer

The invention discloses a reduced-pressure type backflow preventer which comprises a valve body, two check valves with the same structure and a drain valve, wherein a cavity is formed in the valve body, and a water inlet, a water outlet and a drain opening are formed in the valve wall of the valve body; the two check valves are fixedly mounted in the valve body through support pieces and are located at the water inlet and the water outlet respectively; the drain valve is mounted outside the valve body and located at the drain opening; each check valve comprises a valve seat, a valve clack, a rocker and a twins torsional spring; one side of each valve clack is hinged to the upper end of the corresponding valve seat, and rollers are mounted on the other sides of the valve clacks; the rockers are connected with the lower ends of the valve seats through pin shafts; the rollers are propped against the rockers and slide on the rockers; the pin shafts are sleeved with the twins torsional springs; and the spring forc