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The resonant coupling of optical microcavities to waveguides is important in photonic devices. In this paper, a horn-shaped cavity structure is designed on the side of the metal–dielectric–metal waveguide, and the coupling between the cavity and the waveguide is simulated by the finite-difference time-domain method and the coupled mode theory. It is found that the cavity and local modes appear in the horn-shaped cavity. Second, the geometric parameters of the cavity structure are changed, and the influence of the structural parameters on the transmission spectrum is obtained by theoretical analysis. Third, the maximum refractive index sensitivity of the structure is calculated to be 1750 nm/RIU, and the temperature sensitivity is 2.455 nm/°C. Ultrafine particles are placed between the tips of the horn-shaped cavity structure, and the sensitivity of the wavelength shift of the localized mode and the change in the transmittance of the trapezoidal cavity mode to the particle size and the refractive index of the particles are obtained; the nanoparticle sensor is designed by using this characteristic. The horn-shaped resonator structure proposed in this paper provides a high-performance cavity choice for the design and application of micro-nano sensor devices. [ABSTRACT FROM AUTHOR]