<p>Radiative cooling offers a passive and energy-efficient approach to thermal regulation, yet most conventional implementations lack dynamic tunability under varying environmental conditions. Recent advances in switchable radiative cooling materials have shown potential for practical applications, but their transient optical behaviors remain underexplored due to the limitations of conventional spectrophotometers. In this study, a real-time transmittance measurement system is developed that integrates a high-speed spectrometer and a photodiode for dynamic analysis of wet/dry-based switchable radiative cooling films. The system achieves a temporal resolution of 353 ms and ensures measurement reliability through simultaneous monitoring of the light-source intensity. Validation against a commercial UV–Vis spectrophotometer demonstrates high accuracy (maximum error 2.64%), and precision is confirmed through repeatability and reproducibility tests (coefficient of variation 3.57%). Using this system, the transmittance-switching behavior of porous PVDF-HFP films under various conditions is systematically investigated. The results reveal that solvent viscosity, refractive index matching, and film thickness are key factors governing response time and transmittance. Moreover, vacuum-assisted infiltration significantly enhances switching performance, achieving a wet-state transmittance exceeding 85% within 5 min. Overall, this study introduces a robust, time-resolved optical characterization platform and provides a practical methodology for evaluating the dynamic performance of switchable radiative cooling materials in realistic application scenarios.</p>