The water-casting method is one of the most important techniques for the preparation of pinhole-free ultrathin film for air separation. Strong interest exists in the optimizations of preparing routes of the ultrathin film and of the fabrication of the ultrathin-film composite membranes with a variety of polymers. A number of water-casting solvents with high spreadability over water are offered in the present review. Progress made in the inconsecutive and continuous preparations of ultrathin films is reviewed in detail. The oxygen permselectivity, air separation performance, and the stability of ultrathin-film composite membranes are discussed with a detailed citation of references. This review also compares the performances through ultrathin-film composite membranes fabricated by the water-casting method with other methods.