Impacts of substrate membrane storage conditions, which might have special significance in the industry and laboratory research of composite membranes, are rarely reported. Herein, the polysulfone ultrafiltration membrane was chosen as a typical substrate membrane and differences between substrate membranes stored in wet and dry conditions were discussed. An easy-operational hydrothermal pretreatment method was therefore proposed to regenerate the thy substrate membranes. The substrate membranes were heated in deionized water, and the pore morphology was significantly improved after the pretreatment. Three types of additives were applied to improve the effects of pretreatment and the effects of the hydrothermal pretreatment turned to be stronger as the concentration of the additive increased. Membrane structure parameters were investigated and the dry support membranes pretreated with 3.0% or 6.0% molar fraction ethanol displayed approaching structure parameters with wet membranes pretreated with the same conditions, which means the dry substrate membranes were successfully regenerated. Poly(vinylamine)-polysulfone composite membranes were prepared with polysulfone substrates before and after pretreatment, and tested by using CO2/N-2 (15/85 by volume) mixed gas. Composite membranes prepared with wet and dry polysulfone substrates pretreated with 3.0% molar fraction ethanol exhibited similar CO2 permeance, which confirmed that the dry substrate membranes were effectively regenerated.