An optical-fiber gas sensor system based on narrowband scanning light source is proposed. It can complete the concentration detection of various gases mixed in the environment. As usual, traditional fiber gas sensor system requires multiple light sources for detection when there are several different kinds of gases. By using scanning light source, the cost of industrialization gas detection can be reduced. Using an Amplified Spontaneous Emission (ASE) source as foundmental instrument, combining tunable Fabry-Perot filter driven by saw-tooth wave (STW), narrowband scanning light is generated. As the Fabry-Perot tunable filter is a capacitive device, it can cause the linear distortion of the saw-tooth wave. Parallel resonant circuit solutions are proposed to decrease saw-tooth wave linear distortion, thus the stability of light source scanning work is guaranteed. Experimental results show that the system can detect the gas concentration of different components which are mixed with acetylene and ammonia gas. Experimental results also show low error rate.