Based on the theory of all-dielectric metamaterial absorber，a new type of micro-cavity structure absorber for refractive index sensor is designed. Numerical simulations show that the absorber achieves 99.91% and 99.96% absorption at 539 nm and 673 nm，respectively，and is well matched with the Impedance of free space. The full width at half height is 11 nm and at half width is 5 nm，respectively. The absorption mechanism based on Mie resonance is explained by the electromagnetic field distribution at resonance wavelength，and the characteristics of the absorber which is insensitive to polarization angle are analyzed. The changes of absorption peak value and absorption frequency band are studied by changing the structure parameters of the absorber，and the sensing performance of the structure is analyzed. The FWHM，sensitivity S，FOM and FOM * of the two-channel absorber as the refractive index sensor are calculated based on the two absorption peaks as a function of the width of the ring nano-cavity. When the refractive index of the surrounding medium is changed from 1.00 to 1.08，the sensitivity s value can reach 175 nm/RIU and 212 nm/RIU. The sensitivity s value of peak 1 increased from 146 nm/RIU to 201 nm/RIU，while the sensitivity S value of Peak 2 remained stable at about 200 nm/RIU when the width of the annular nano-cavity increased from 20 nm to 100 nm. The FOM value of peak 1 increased slowly from 8.2 to 23.6，while that of Peak 2 was 65.3 because of its narrow frequency band. In addition，the FOM * values of the dual-band absorber at 539 nm and 673 nm resonant wavelengths are calculated to be 17 265 and 59 902，respectively. The absorber can achieve double-channel high absorption in the visible band，and has the advantages of high sensitivity and narrow band when used as a refractive index sensor. The microcavity structure of the absorber has potential applications in future Environmental monitoring and photoelectric detection.