Niobium nitride (NbN) nanowires are commonly used as photosensitive materials for superconducting nanowire single-photon detectors (SNSPDs). Their optical properties are the key factors influencing the performance of SNSPD. According to the experimental data and simulation results, in this paper we systematically study the optical properties of various niobium nitride nanowire detector device structures, and characterize the reflection spectra and transmission spectra of the following four device structures:1) Backside optical devices with SiO ₂as the antireflection layer, 2) backside optical devices with SiN as the antireflection layer, 3) front-facing optical devices with Au+SiN as a mirror, and 4) front-facing optical devices with distributed Bragg reflector (DBR) as a mirror. The NbN films with different thickness are grown on the basis of the four device structures, and the absorption efficiencies of the NbN films with different thickness are observed. The relationships between the optimal NbN thickness and the optical absorption rate for different device structures are found as follows:The maximum absorption rate of NbN on the SiO ₂antireflection layer is 91.7% at 1606 nm, while the absorption rates of the remaining structures at the optimal thickness of NbN can reach 99% or more. The absorption rate of the SiN device, the Au+SiN device and the DBR device are 99.3%, 99.8% and 99.9%, respectively. The measured results and simulation structure of DBR device are analyzed. These results are of significance for guiding the design and development of high efficiency SNSPD.