In this study, a series of polycarbonate/polyacrylonitrile (PC/PAN) composite membrane has been successfully fabricated by vapor-induced phase separation and in-situ casting process. The effect of the casting solution concentration on the membrane morphology was investigated. The morphology of the PC/PAN composite membranes was characterized by SEM. The effect of the skin layer thickness on the water flux was measured at a fixed pressure. Compared with the traditional asymmetric PAN membrane with a dense skin layer and porous support layer, the PC/PAN composite membranes prepared by in-situ casting process effectively prevent the skin layer formation, resulting in the water flux increases.