A carbon monoxide (CO) gas sensor based on copper oxide/polyaniline composite membrane-coated photonic crystal fiber is proposed. The Mach-Zehnder interference structure is formed by fusion splicing a standard single-mode fiber to a solid photonic crystal fiber. The copper oxide/polyaniline composite material is coated on the surface of the photonic crystal fiber, and then the purpose of detecting carbon monoxide can be achieved. The results show that a uniform layered film is formed on the surface of the fiber with a thickness of about 2 μm. A high sensitivity of 17 pm and a good linear relationship are achieved in the range of CO volume fraction of 0~75×10 -6. The response time and recovery time are 80 s and 110 s, respectively. The sensor has an advantage of low cost, simple structure, and easy to manufacture.