In order to improve the sensitivity of optical index sensor, a metal-medium-metal（MIM） waveguide coupled elliptic eight shape cavity is designed. Firstly, the principle and key performance index of the sensor structure are analyzed theoretically. Then, the simulation structure is constructed by using finite element analysis simulation software to study its sensing performance. The simulation results show that when a=540 nm, b=340 nm, g=10 nm, l=70 nm, the sensitivity of the designed sensor can reach 1 323 nm/RIU, and the corresponding quality factor can reach 16.