Carbon dioxide is absorbed by de-ionized water or NaOH aqueous solution in the unsteady-state membrane absorption process. A theoretical model has been developed to describe the mass transfer behavior in the liquid phase, in which the effects of membrane structural characteristics are investigated. The concentration profiles in the liquid phase are calculated as a function of time. When the membrane porosity is relatively high or the pore size is relatively small, the solute concentration profile near the membrane surface can get homogeneous instantly due to the short distance between adjacent pores. In this case, the existence of the porous membrane has less effect on the mass transfer process. However, when the membrane porosity is relatively low or the pore size is relatively large, the distance between the adjacent pores is large, so the concentration profile near the membrane surface is hard to get homogeneous during the absorption process. Therefore, the concentration profile can be influenced significantly by the membrane structural characteristics, which means that the membrane structure has a significant effect on the mass transfer in liquid phase. Moreover, the chemical reaction in the liquid phase makes it difficult for the concentration profile near the membrane surface to get homogeneous. The disturbance in the liquid phase caused by the gas flow and pressure fluctuation is also taken into account in the model, and the model results agree well with the experimental data.