Abstract: Electrochromic (EC) windows is a new type of small windows. The visible light transmission rate and the solar heat gain coefficient can be actively adjusted according to the demand. To explore the energy-saving characteristics of the application of EC windows, this paper took an office in Beijing as a case study and took the EC windows sold in the market as the research object in the market.The EnergyPlus simulation was used to study the air conditioning with different control methods and different control thresholds of the EC windows. Due to different demand characteristics of EC windows in different seasons, EC windows should adopt different control strategies in different seasons, and the selection of control parameter threshold also has an important impact on building energy consumption.The EC windows have the lowest energy consumption in the cold season, the illumination and in the heating season with the glare control. Compared with the ordinary windows, the high permeable low emissivity (low-e) windows and the low permeability low-e windows, the energy saving is 13.2%, 11.0% and 11.7%, respectively. The results of this study can provide reference for energy saving applications of EC windows in China.