This paper presents a novel system for detecting concentration of the CH4 and CO gases, which exist widely in the coal industry. The system applies micro blazed grating to split the collimated light and may get the infrared absorption light with certain wavelength and bandwidth for the gases detection. The schematic diagram, calculation, signal processing, and setup were described in details. Experimental results indicate that the detection system has the ability to detect methane and carbon monoxide with maximum error of 0.169% and 58.3ppm, stability of 0.76% and 0.22%, respectively.