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. This system has characteristics such as full optical fiber light way and simple structure. 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. The measuring sensitivity and stability are also further enhanced. Experimental data and results show that there is a high measuring sensitivity at 0.01%, and the measuring range goes beyond 5%.