Compared with the mature design in near-infrared band, the lower photon energy in the 3-5 μm wavelength range puts forward higher requirements for the design of broadband optical absorption characteristics of superconducting nanowire single-photon detector (SNSPD). Therefore, a design method of broadband optical absorption characteristics of SNSPD in mid infrared band is proposed. Taking the ultra narrow nanowire structure SNSPD loaded with SiO2/Au reflective cavity as an example, on the basis of realizing impedance matching at two target wavelengths by optimizing the thickness of three dielectric layers in the upper and lower cavities, the ratio of total dielectric layer thickness to resonant wavelength is introduced as an index to evaluate the bandwidth characteristics at each resonant wavelength, so as to better achieve the balance between impedance matching accuracy and bandwidth characteristics, thus, the purpose of double broadband coupling resonance to expand the overall absorption bandwidth is achieved. The numerical results show that the proposed SNSPD can absorb at least 50% of the incident light whose electric field component is parallel to the nanowires in the wavelength range of 2928-4856 nm.