To address the bottleneck issue that the photon detection efficiency (PDE) of traditional planar silicon photomultipliers (SiPMs) is limited due to high surface reflectivity, this study proposes a solution based on micro-nano etched arrays. By designing photosensitive structures of inverted pyramid arrays and inverted hemispherical arrays, which reconstruct the photon propagation path and modulate the interface roughness, the surface reflectivity is significantly reduced. Multiphysics coupling simulation is employed to quantitatively analyze the reflectivity, absorptivity, and optoelectronic characteristics of the two structures. Simulation results show that under the conditions of a hexagonal arrangement of circular avalanche photodiode units (with a unit diameter of 13 μm and a spacing of 3 μm), the average reflectivity of the two micro-nano structures under natural light irradiation in the wavelength range of 250–1100 nm is reduced to 19.48% and 29.51%, respectively, which are significantly lower than the 43.27% of the planar structure. Meanwhile, their average absorptivity in the 250–1100 nm range is increased to 79.35% and 69.29%, respectively, which are notably higher than the 36.44% of the planar structure. At a wavelength of 500 nm, the PDE of the two structures is increased by 61.74% and 22.90% respectively compared with that of the planar structure. This study can provide a theoretical basis for the design and fabrication of SiPMs with high PDE.