Unlike the high absorptivity design at a specific wavelength, the optical absorption design of superconducting nanowire single-photon detectors in a wide wavelength range of 3?5 μm necessitates an enhanced balance between the peak absorptivity and in-band flatness. Herein, first, an initial front-illuminated device model based on the asymmetric Fabry-Pérot (F-P) cavity structure is stacked using ultranarrow NbN nanowires, SiO₂ cavities, and distributed Bragg gratings (DBRs). Second, the three thicknesses of the SiO₂ cavities, a high refractive index layer and a low refractive index layer in the DBR are considered as the optimization variables, and the minimum light absorptivity in the wavelength range of 3?5 μm is set as the optimization objective. Finally, the initial device structure is optimized using the particle swarm optimization algorithm. Results show that compared with the design of double-wavelength coupling, the design of a single-layer NbN nanowire detector based on the asymmetric F-P cavity structure and high-contrast DBR can afford improved minimum optical absorption and in-band flatness by 40.2% and 59.2%, respectively. Based on this, a double-layer NbN nanowire layout can enable a further enhancement in the minimum optical absorptivity and increase the maximum absorptivity to more than 0.97, a value equivalent to that obtained using the double-wavelength coupling method.