This study proposes a D-shaped photonic crystal-fiber sensing structure for exploring the physical mechanisms of sensitivity improvement. The structure is based on surface plasmon resonance enhanced using gold nanoparticles. The gold particles are modified on the surface of a gold film coated on the D-shaped polished surface. The free charge on the gold nanoparticles interacts with the induced charge in the metal film, improving the intensity and sensitivity of surface plasmon resonance. The effects of the structural parameters on the sensing performance are investigated numerically using the finite element method assuming an anisotropically perfect matching layer. The D-shaped photonic crystal fiber/Au film/gold nanoparticle sensor with a gold nanoparticle diameter of 12 nm, a relative spacing between the gold nanoparticles of 3.2× the nanoparticle diameter, a gold film thickness of 45 nm, a distance between the fiber core and polishing surface of 3.0 μm, an air hole spacing of 2.0 μm, a large air hole diameter in the outer layer of 0.75× the air hole spacing, and a small air hole diameter in the inner layer of 0.80× the large air hole diameter, the linearity is 0.97 and the refractive index sensitivity is 6240 nm/RIU in the refractive index range 1.370?1.385. The linearity is 0.99 and the refractive index sensitivity is 13640 nm/RIU in the refractive index range 1.385?1.400. In the refractive index range 1.370?1.400, the average refractive index sensitivity reaches 9940 nm/RIU, 1.27× that of a D-shaped photonic crystal fiber/Au film sensor. The proposed sensing mechanism and structure are expected to assist research and applications in areas such as biomolecular detection.