The vapor pressure of desiccant solution is one of the most important parameter of liquid desiccant cooling system. It dominates the performance of this system. This paper selects two typical models—NRTL equation and simple mixed model—to analyze the formation mechanism of the vapor pressure and the role it plays in dehumidification process. The vapor pressure of mixed LiCl-CaCl2 solution is calculated according to these two models. Then the models are verified through measuring the vapor pressure of various mixture ratios and various mass concentrations of the mixed solution under different temperatures. Finally the research considers the mixed solution of 50% LiCl and 50% CaCl2 as the best mixture ratio. Simple mixed model can be adopted when the solution is at low mass concentration while the models should be amended to conform to the actual situations when the solution is at high mass concentration. The modified model is able to make an estimation or prediction on vapor pressure under different conditions which offers a theoretical basis to determine the mixed liquid desiccants in the design of liquid desiccant cooling system.