In this paper, an amperometric gas sensor based on proton exchange membrane (Nafion) was fabricated for H₂S detection at room temperature. Pt-Rh nanoparticles loaded on carbon fibers, as sensing electrode, was prepared by chemical reduction method, and membrane electrode assemble (MEA) was fabricated using hot-pressing method. X-ray diffractometry (XRD) and field emission scanning electron microscopy (FESEM) were used to analyze the structure and morphology of the fabricated electrode material. The effect of the weight ratio of Pt to Rh on the performance of the sensor was investigated and the results showed that the sensing performance was best when the ratio of Pt to Rh was 1:1. Moreover, the sensitivity had been significantly improved due to the use of carbon fibers pretreated by nitric acid, increasing from 0.158 mu A/ppm to 0.191 mu A/ppm. The present sensor could detect H2S at levels as low as 0.1 ppm with a -20 nA response and had a good linear relationship in the range of 0.1 to 200 ppm H₂S. In addition, this amperometric H₂S sensor displayed fast response-recovery rate as well as excellent selectivity and stability.