The phase equilibrium curves of aqueous LiCl solution and air under isothermal and adiabatic absorption processes were plotted respectively to design the absorption towers which are used in liquid desiccant air conditioning systems. The researches showed that the former curves were influenced by the temperature of the solution while the latter were influenced mainly by the concentration of the solution. The latter curves also showed that the latent heat of condensation of water vapor and the differential enthalpy of dilution of the solution caused the absorption system to reach the heat and mass balance situation much faster than isothermal process, which meant that lower height tower could be adopted for adiabatic processes. The required solution’s volumetric flux was calculated using the phase equilibrium curves and the results showed that the mass and heat balance relations should be considered simultaneously to get reasonable results, while only mass balance relations were used in traditional calculating methods.