Compared with gradient index (GRIN) gas cell,the sensitivity of prism gas cell is easy to be adjusted and the interference noise is apt to be suppressed. Based on the Beer-Lambert law,detection of various volume fraction (0-20 %) of methane is completed by subtraction of background and harmonic detection method,as the atmosphere surroundings is treated as background. The direct absorption spectra of various volume fraction are measured using GRIN gas cell,combined with available distributed feedback laser diode (DFB-LD),where the R5 line of the 2ν3 band of methane is selected as absorption peak. The system is tested online during gas mixing process and the linear relation between system indication and volume fraction variation is validated,also the stability and dynamic response characteristics is confirmed by experiments. The system sensitivity can be adjusted according to volume fraction level of various field environments by changing the prism distance and using step motor. The system can be applied to various fields and can be adopted as monitoring instrument for coal mine tunnel and natural gas pipeline.