Super material perfect absorber is very important in the research of refractive index sensor. The finite difference time domain method is used to simulate the surface plasmon structure. According to the impedance matching principle, the parameters of the top layer structure are optimized. The absorption rate of the structure to the light wave is 99.6% , when the wavelength is about 12 micron. The sensitivity of the structure is 4 008 nm/RIU, and the structure can be used to measure the refractive index of gas.