Abstract: The hydrophilicities,water vapor permeation of a series of modified polyimide(PI) membranes,and the apparent viscosities of their mixed solutions were investigated.The results show that the membrane hydrophilicities and water vapor permeation increased with increasing of quaternary ammonium salt,at the same time,water vapor permeation is affected by membrane thickness and relative humidity,blend solution viscosities raise with increasing quaternarg ammonium salt in PI,and an enhancement phenomena of blend solution is observed.The dehydration properties of blend hollow fiber membranes are tested and it is found that increase of the feed pressure and the sweep flux favor the dehydration performances of blend membranes.Although the properties of membranes are not always improved with the(increasing) the membrane hydrophilicities,but when increase the quantity of quaternary ammonium salt(appropriately,) the dehydration properties of blend hollow fiber membranes would be improved effectively.