In this study, a highly sensitive and narrow–spectrum enhanced multiparameter gas sensor is proposed based on Fano resonance in an asymmetric metal–dielectric–metal (MDM) waveguide. A side-coupled MDM structure comprising an upright rectangular cavity is initially investigated under different gaps. An assumed junction model is used to analyze the physical mechanism of a through coupled system (gap = 0) with an ultralow minimum transmission coefficient and higher refractive index sensitivity compared to other cases. Becausethe transmission spectrum of through-coupled structure is relatively broad, a ring resonator was added on the upper side of straight waveguide to generate an ultranarrow resonant mode as a discrete state that interferes with the previous broad mode serving as a quasicontinuum state to afford Fano resonance. Consequently, its full width at half maximum decreases by 420% from 496 to 11.56 nm. The response rate and detection difficulty of detection of y-asymmetric plasmonic nanostructures used in this study are exceed those of x-asymmetric plasmonic nanostructures. A multiparameter gas sensor can be developed by filling hydrogen- and methane-sensitive films in corresponding cavities of the plasmonic nanostructure. The sensitivities of methane and hydrogen can reach −2.863 and −0.265 nm/%, respectively, after optimizing relevant structural parameters. The proposed method provides an effective approach for designing narrow-spectrum enhanced and multichannel nanophotonic devices.