Abstract: A surface plasmon resonance refractive index sensor based on the coupling structure of gold nano cones and a gold film with a SiO₂ film as spacer-layer is designed. The surface plasmon resonance modes in the composite structure are studied by using the Finite Difference Time Domain method. The composite structure can stimulate not only localized surface plasmon, but also propagating surface plasmon. The energy of the incident electromagnetic wave is partially coupled to the localized surface plasmon through a single gold nano cone, and partially coupled to the propagating surface plasmon through a grating of gold nano cone array. The reflection spectra of the composite structure are simulated in the refractive index range of 1.30 to 1.40. It is found that the resonance wavelength has a linear relationship with the refractive index of the analyte, and the reflectivity at the resonance is almost zero due to the strong resonance coupling between localized and propagating surface plasmon. In addition, the full width at half maximum of propagating surface plasmon resonance mode is very narrow when the geometric parameters of gold nano cone are optimized. The sensitivity and figure of merit reach 770 nm/RIU and 113 RIU⁻¹ respectively, and it has good refractive index sensing performance. The designed composite structure is expected to be widely used in the field of biochemical detection.