A novel surface plasmons (SP) sensor system for gas with high-refractive index resolution is designed. The method of reflective wavelength detection is used in the system of gas sensor that works in wavelength of optical fiber communications. The sensor is based on a metal grating with a spacer between the periodic strips and a metal film (MGS). The SP excited by gratings, is constrainted in the waveguide layer between the metal grating and the metal coatings, which enhances the SP effect, makes the SP resonance peak seasitive to the charge of gas index. This gas sensor has higher sensitivity compared with metal grating sensor. The computation demonstrates that the average wavelength resolution over the range of 1.000~1.001 is approximately 3050 nm/RIU, which is twice more than metal grating-based gas sensors. The minimum resonance peak reflectivity and the full width at half maximum (FWHM) values are only 0.25% and 4.2 nm, respectively. The gas sensor based on MGS has the characteristics of high sensitivity and resolution, safe, ease of integration, enabling real-time detection and remote monitoring. It has very important application value for gas refractive index detection.