Condensing dehumidification method is widely applied to humidity control in manned spacecraft cabin. Under the micro-gravity condition, the condensate water has to be removed actively by additional water/air separation and water recycling processes. This paper presents a novel three-fluid membrane dehumidification method for spacecraft humidity control, which is expected to replace the condensing method. The cold water provides a low water vapor partial pressure to drive the moisture transfer across the membrane from the humid air, while the hot water maintains the humid air temperature above the dew point to prevent the possible dewing. The diffusivity of water vapor in membrane is first determined by an experiment with the help of the particle swarm optimization (PSO) algorithm. With the experimentally determined diffusivity, the three-fluid membrane dehumidification is studied experimentally and theoretically, and the mathematical model reaches a good agreement with the experimental results. The novel dehumidification method is experimentally validated to be feasible; that is to say the dehumidification can be realized and meanwhile there is no dewing on the outer surface of the membrane. The heat and mass transfer performance is also studied under various operating conditions. The three-fluid membrane dehumidification shows superior thermodynamic performance compared with the condensing method.