Infrared fire detector, also known as linear infrared beam fire smoke detector, works on the principle that smoke particles absorb or scatter infrared light, thus changing the intensity of infrared beam. Its working principle is similar to that of light-shielding photoelectric smoke detector, except that smoke does not need to enter the sampling room of point photoelectric smoke detector, and smoke anywhere in the protected space may attenuate infrared beams. The beam transmitter and receiver are two independent parts, and there is no photosensitive room. As a measuring area, the optical path is exposed in a protected space and extended many times. When there is no smoke in the measurement area, the infrared beam emitted by the transmitter is received by the receiver, and then the system is adjusted to the normal monitoring state. If the smoke diffuses to the measurement area, it will absorb and scatter the infrared beam, thus weakening the optical signal reaching the receiver. When the signal decays to a certain extent, the receiver will send out an alarm signal. The invention has the advantages of large protection area and high installation position, and is suitable for the protection of large spaces. There are two types of linear infrared beam smoke detectors: opposite and reflection.