Thermochromic smart windows can passively adjust their transmittance response to the temperature, and their phase transition temperatures largely determine the response speeds, which further influence the indoor thermal environment. This study develops a three-dimensional CFD simulation model to investigate the impacts of thermochromic materials with different phase transition temperatures on the thermal performance and thermal comfort of the indoor environment. Firstly, two types of thermochromic hydrogel materials with phase transition temperatures of 29.8 ℃ and 33.9 ℃ were prepared experimentally. Then the measured optical and thermal properties of hydrogel material were input into the numerical model to simulate the all-day performances. It is suggested that the hydrogel window with a lower phase transition temperature changed color earlier and more effectively reduced indoor heat accumulation. Compared to the normal glass, the 29.8 ℃ hydrogel window achieved a maximum floor temperature reduction of 0.57 ℃. After the phase transition, the indoor PMV value ranged from −0.45 to 0.57, and the maximum PPD value was decreased by 1.1% compared to the transparent window. This study contributes to the selection of hydrogel thermochromic smart windows for building applications.