Abstract: Moisture absorption by falling film liquid desiccant is certainly a problem of heat and mass transfer of two phase flow. Heat effect produced in dehumidification makes process air and falling film warmed.Experimental practice shows that these two flows obtain different portions of heat, thus the coefficients of heat transfer for liquid side and gas side are different.While the moisture removed from process air is the same as the water absorbed by liquid desiccant, so there is aunique coefficient of mass transfer for absorption. A detailed mathematical model describing the coupled heat and mass transfer is presented and solved numerically. The calculation results indicate that for laminar flow of both process air and falling film, the Nusselt number of liquid side is about 2.84, while that of gas side is about 6.54, and the Sherwood number at the gas-liquid interface is about 5.40.In addition, theoretical analysis is conducted under given ideal conditions.It is demonstrated that theoretical Nusselt number for liquid side is 3.636.For turbulent flow of process air, Nusselt numbers and Sherwood number all increase with the increase of Reynolds number.It is concluded that the coefficients of heat and mass transfer are dependent on the thermal physical characteristics of liquid desiccant and the flowing state of process air.