Gaseous impurities contained in hydrogen (H2) profoundly affect the performance of hydrogen proton-exchange membrane fuel cells. We demonstrate the utility of cavity-enhanced Raman spectroscopy as a unique approach for detection of gaseous impurities. A dense-pattern multipass cavity which is composed of four spherical mirrors placed in a Z-shaped configuration is used to enhance the Raman signal by extending the laser-gas interaction length. A total of 85 spots are identified on the 2-inch-diameter front (or rear) mirror, which indicates that 510 beams exist in the cavity. Detection limits of the impurity gases, including oxygen (O2), nitrogen (N2), carbon monoxide (CO), carbon dioxide (CO2), methane (CH4), ammonia (NH3), and hydrogen sulfide (H2S), reach sub-ppm- and ppb-levels at a total pressure of 0.1 and 2.5 MPa, respectively. This satisfies the detection requirements according to the maximum allowable concentration for these gases. Our cavity-enhanced Raman spectroscopy (CERS) apparatus can simultaneously measure multiple gases with high sensitivity and selectivity with no sample destruction. It has excellent application prospects in gaseous impurity analysis for the quality assessment of gaseous energy.