A novel fiber sensor for measuring gas concentration is reported, which is based on the transient regime of a Er-doped fiber laser. A Sagnac fiber loop and a FBG that also acts as the sensing element form the linear fiber laser cavity. By adjusting the central wavelength of the grating mirror, the oscillating wavelength is set to 1531.56 nm, which is a strong absorption peakof C2H2. The gas concentration can be obtained by measuring the laser build-up time, which is closely related with the cavity loss and therefore determined by the gas concentration. In the present experiment, the sensor's sensitivity and resolution can be adjusted simply by controlling the pumping level. When the pump pulse is at high level and low level of 25 mW and 5.9 mW, respectively, a sensitivity of 100 ppm/μs and resolution of 20 ppm are achieved.