Phase change material (PCM) has gained great attraction for being widely used in tunable photonic devices due to its nonvolatility, excellent scalability, high speed, and tunable properties. However, the unclear source of the phase-change characteristics limits PCM for true device-level integrated circuits with high configuration and multifunctional attributes, especially in the mid-infrared (MIR) region. In this paper, the inherent dependences of phase-change characteristics on both the microstructure and bonding environments of the (VO2) x (Sb2Se)100–x film are disclosed for low-loss and nonvolatile tunable photonic devices. (VO2) x (Sb2Se)100–x films possess a tendency that the crystallization temperature, 10 year data-retention temperature, and optical band gap of (VO2) x (Sb2Se)100–x films increase with the increase of VO2 content. The improvement in thermal stability of (VO2) x (Sb2Se)100–x is ascribed to the complex bonding environment consisting of Se–V and Sb–V bonds with the large bonding enthalpy, which disturbs the crystallization. The symbolic value for the contribution of resonant bonding ζ is derived by the real part ε1 and imaginary part ε2 of dielectric functions with a wavelength ranging from 1.7 to 25 μm. With increasing VO2 content, a large structural distortion in the VO2-rich phase leads to a reduction in the level of resonant bonding, which accounts for a decrease in the optical refractive index contrast. These results shed light on intrinsic material science and engineering to the ongoing search for other phase-change materials for tunable photonic devices in the MIR region.