The backflow preventer consists of two isolated check valves and a hydraulically actuated drain valve, with the first check valve housing a filter screen. Due to the local head loss of the check valves, the pressure within the intermediate chamber remains lower than the inlet pressure. This pressure differential keeps the drain valve closed, ensuring normal pipeline water supply. During abnormal pressure conditions (i.e., when outlet pressure exceeds the intermediate chamber pressure), even if both check valves fail to seal against reverse flow, the safety drain valve automatically opens to discharge backflow water. This creates an air barrier, ensuring the sanitary safety of upstream water supply.