We succeeded in generating nanopores in a cellulose acetate (CA) matrix under water pressure using copper nitrate (Cu(NO3)2·4H2O) as the additive. The nanopores were produced when the polymer matrix was subjected to isostatic water pressure adding copper(II) nitrate into the CA membrane. The polymer chains were weakened by the plasticization effect of copper(II) nitrate present in the CA membrane. We confirmed that the pore properties were readily changed by water pressure as external physical forces with additive content. The water flux of the CA composite (CA/ Cu(NO3)2·4H2O) membrane improved with increasing water pressure and additive content. For the CA membrane without the additive, the water flux was not measured from 1 to 8 bar water pressure. In contrast, the water flow of the polymer film with the additive varied from 95.3 to 205.75 L/m2h, depending on the water pressure and additive content.