Surface plasmon resonance(SPR) technology has been widely used in refractive index measurement in the external environment. In this paper, a symmetrical planar waveguide structure is designed. The relationship between SPR effect and the refractive index of the external environment is numerically simulated by using the finite difference time domain method. The selection of metal materials and the length of sensing area are optimized. The effects of modes of different orders on the sensitivity of the sensor are studied. The simulation results show that when the refractive index of the external environment is 1.38, compared with the sensitivity of 4100 nm/RIU under the base mode condition, the sensitivity of the third-order mode sensor is increased to 6209 nm/RIU, and the maximum sensitivity is increased by 51%. When the ambient refractive index is 1.34-1.38, the average sensor sensitivity increases from 2900 nm/RIU to 4025 nm/RIU.