The unique properties of polyvinyl alcohol (PVA) and polysulfone (PSf), such as good membrane-forming ability and adjustable structure, provide a great opportunity for CO2-separation membrane development. This work focuses on the fabrication of PVA/PSf composite membranes for CO2/N2 separations. The membranes prepared by coating a 7.5 wt% PVA on top of PSf substrate showed a relatively thin selective layer of 1.7 µm with an enhanced CO2/N2 selectivity of 78, which is a ca. 200% increase compared to the pure PSf membranes. The CO2/N2 selectivity decreases at a rapid rate with the increase of feed pressure from 1.8 to 5 bar, while the CO2 permeance shows a slight reduction, which is caused by the weakening of coupling transportation between water and CO2 molecules, as well as membrane compaction at higher pressures. Increasing operating temperature from 22 °C to 50 °C leads to a slight decrease in CO2 permeance, but a significant reduction in the CO2/N2 selectivity from 78 to 27.1. Moreover, the mass transfer coefficient of gas molecules is expected to increase at a higher velocity, which leads to the increase of CO2 permeance at higher feed flow rates. It was concluded that the CO2 separation performance of the prepared membranes was significantly dependent on the membrane operating parameters, and process design and optimization are crucial to bringing CO2-separation membranes for industrial applications in post-combustion carbon capture.