A long optical path gas monitor is open-typed or close-typed. The monitor includes an optical generation part and a signal processing part. Said optical generation part comprises an emitter, a receiver and multi-group concave mirrors or prisms used to form enough optical path between the emitter and the receiver. Said signal processing part includes an optical fiber (21), a spectrometer (22), a scanner (23), an optoelectronic detector (24) and a computer (25), which are connected sequentially. The input end of the optical fiber (21) is connected to the receiver.