Phase change material applied in the glazing structure can improve its thermal performance, which has effect on its optical performance. A model to determine thermal and optical performance of a phase change material–filled double-glazing unit was developed, and the effect of optical parameters of phase change material was numerically investigated. The results show that the effect of the refractive index of phase change material on temperature time lag, temperature decrement factor, solar transmittance, solar absorptance, and reflectance of a phase change material–filled double-glazing unit is weak when the refractive index of phase change material is lower than 1.95; however, the effect of the extinction coefficient of phase change material is violent. The liquid phase change material with large extinction coefficient is not recommended from the viewpoint of light utilization, but the large extinction coefficient of solid phase change material is accepted from the viewpoint of energy utilization.
