A plasmonic perfect absorber based on the ring hole and slot waveguide in three different structures is proposed as a sensor and simulated using the three-dimensional finite-difference time-domain numerical method for perpendicular incidence of a plane wave light within 900 to 1900 nm wavelength range. The simulation results show that two resonance peaks emerged in the absorption spectrum of the devices. To investigate the sensing capability of this absorber, the ring holes and slot waveguides on the upper surface of the structures are filled with analytes with different refractive indices. It is observed that the resonance peaks are redshifted with increasing the refractive index of the analyte. The sensitivity of the first and second resonance peaks for structure 1 is 260.7 and 154.4  nm  /  RIU, respectively, for structure 2 are 303.4 and 102.7  nm  /  RIU, respectively, and for structure 3 are 261.9 and 103.2  nm  /  RIU, respectively, which indicates the good ability of the devices to detect analytes with refractive indices close to each other.