A mathematical model is established to describe a hollow fiber membrane separator by binary gases which including water vapor under low feed gas pressure and vacuum operation. The permeability of water vapor is described by resistance model in each differential unit and the boundary resistance caused by concentration polarization is taken into consideration. Industrial gas acetylene dehydration by PSF hollow fiber composite membrane is used to test the model. The simulated results show good agreement with the experimental data. The calculated results will bring 50% error of the water vapor composition of the retentate gas when concentration polarization is ignored. The permeability of water vapor is decreased slowly along the fiber from the entrance of the feed gas to the exit, mainly because of the water vapor partial pressure changes with the length of the fiber. The influence of concentration polarization on separation is also discussed by this model. The actual separation factor alpha H2O/C2H2 less than 1/5 to the intrinsic separation factor of the composite membrane due to the concentration polarization. The concentration polarization of the feed side boundary has little influence on the membrane separation process. The permeate side boundary resistance controls the whole mass transfer in the separator.