Based on law of Beer-Lambert, a novel optical fiber gas sensing system is designed to determine the concentration of gas. The narrow-band filtering characteristic of optical fiber Bragg grating is used by this system to produce the narrow spectrum light signal which is wanted in difference absorption measuring. The system has full optical fiber structure, which is mainly composed by super luminescent emitting diode (SLED), optical fiber of 10km, optical fiber gas detector, optical fiber isolator, optical fiber coupler network, optical fiber detecting grating, photoelectric detector and data processing unit. The aspheric objective optical fiber collimator is adopted by this system to compose optical fiber gas sensing detector, which enhances the optic stability of the detector, diminishes the injection loss and makes the echo signal of the optical fiber detector greatly diminished. Experimental data and results show that this system has its feasibility and it can be used for real time monitoring of gas concentration in the domains of colliery and natural gas station, etc.