A new concept called “ideal dehumidification efficiency” was proposed to evaluate the performance of the liquid dehumidification system based on the laws of conversation of mass and conversation of energy. The effects of mass flow rate, inlet temperature and inlet quality concentration/humidity of both humid air and lithium chloride solution were analyzed in detail. It was found that although the increase in solution mass flow rate or decrease in air mass flow rate can increase the dehumidification efficiency, the regularities were different. The relationships between dehumidification efficiency and air inlet humidity were also studied at five different liquid-air ratios. Compared to the existing research, the results obtained in this paper were much more reasonable and precise.