Infrared multifrequency flame detector

The invention provides an infrared multifrequency flame detector which comprises three infrared pyroelectricity sensors, three three-level amplifying circuits, a thermo-electric pile temperature detector, a one-level amplifying circuit, an AD conversion circuit, a microprocessor circuit, a relay output circuit, a 4-20 milliampere temperature output circuit and a detector power circuit, wherein the output ends of three infrared pyroelectricity sensors are respectively connected to the input ends of three three-level amplifying circuits; the output end of each pyroelectricity temperature detector is connected with the input end of the one-level amplifying circuit; the output ends of three three-level amplifying circuits and one-level amplifying circuit are respectively connected with the input end of the AD switching circuit; the output end of the AD conversion circuit is connected with the microprocessor circuit which is connected with the input end of the relay output circuit so as tooutput a relay control si