This paper realizes the four-channel optical fiber combustible gas monitoring system by using the tunable diode laser absorption spectroscopy (TDLAS) and light split ting technology.The monitoring of the system is the separation type.The passive sensing probe and active instrument are connected by the optical fiber.The light signal containing concentration inform ation transfers to the instruments to be processed through the optical fiber.The sensing probe which is only composed of the gas chamber and optical component w ithout any circuit can guarantee the intrinsic safety and the anti-interference ability of the probe.The system can realize the warning forecast of the remote,long-term online distributed gas con centration.The paper also designs a phase-lock multiplication demodulating circ uit,which can analyze the weak signal and solve the phase delay caused by the fi ber transmission and find a solution to the deviation of the results.The monitor system emplaces 71optical fiber methane sensing probes in a large oil depot re servoir area.The long-term experiment results show that the measurement results of the system are not influenced by the length of the optical fiber,and the sys tem can get a correct detection of gas concentration value in the 0－5% (0－100LEL) measuring range.