The gas monitor with optical fiber is developed in this paper based on the principle that the absorption light changes with gas intensity under absorptive peak wavelength. The LED of driving light source is selected by the absorptive peak wavelength of measured gas. The light source driving circuit with the different frequency is used to detect the varied gas in real time. The techniques of phase-sensitive detected and ratio between measured signal and reference signal is used to ensure measurement accuracy. The examination system chooses low loss 50cm-long quartz optical fiber as long distance transmitting channel. The light source adopts low power pulse modulation semiconductor LED. 50cm-long absorption gas room is used to ensure measurement sensitivity. The system adopts the technique to carry out scheme combining optical fiber sensing technique with computer data handling technique. Uniting sensing light signal and stimulating light power transfer and measured parameter sensing, the advantages of optical fiber system, micro-electron electric circuit and computer compatibility can be sufficiently realized. The entire measurement system can attain light, the engine and electricity integration, and realize multi-parameter and multi-function. The experiment result of methane and acetylene intensity indicates that the system has higher detected sensitivity and detected accuracy. The system is not only used to detect the methane and acetylene intensity, but also to detect other gas intensity under a little structure parameter change.