Ventilation and air humidity are two important factors determining the thermal comfort of indoor environment. Liquid desiccant dehumidification has become one of the widely used dehumidification technologies with high efficiency and energy storage capability. Different from the conventional packed beds dehumidification, membrane-based dehumidification systems overcome the problem of liquid droplets from crossing over. Heat and mass transfer phenomena of the membrane-based dehumidification are much more complicated than the commonly well-investigated packed columns. Based on the literature studies, a simplified steady-state counter-flow mathematical model has been developed in this paper for a single layer membrane unit. The theoretical model can be used to carry out performance analysis and system design. The governing equations were solved iteratively by finite difference method. The performance analysis has been completed for a small-scale membrane-based dehumidification module consisting of 8 air channels and 8 solution channels. The influences of main design parameters on dehumidification effectiveness have been evaluated, including air and solution inlet conditions, air flow rate (NTU) and solution to air mass flow rate ratio (m*). The numerical results indicated the capabilities of the membrane-based unit to provide the dehumidified air under various climate conditions. The system NTU and solution to air mass flow rate ratio (m*) are two key parameters impacting the dehumidification performance, and they affect each other positively. Higher sensible and latent effectiveness can be achieved with larger NTU and m*. Suggestions for the system design and operation have been provided with performance evaluations.