Based on pyroelectric infrared sensor working principle, the passive pyroelectric infrared alarm system is designed, which is mainly used for safety of residence house to detect whether outsiders enter or not. This system is made of pyroelectric infrared sensor, Fresnel lens and monitoring circuits. The concealment of infrared is well applied on the circuits , amplifying and filtering circuit , infrared signal processing circuit are designed, then the voice chip is utilized to simulate alarm voice for warning thieves and burglars .After many experiments, This system has sensitive response , high anti-interference ability and safe and reliable performance.