A polymer membrane has been developed as a competitive technology in the field of water purification and treatment. The coupling of nanomaterials and membrane technology might inspire some interesting and extraordinary harvests in the water-treatment application of polymer nanomaterials composite membranes. We discuss the behaviors of different types of nanomaterials in the process of classical phase inversion and the resultant effects on the structure performance relationship of an as-prepared polymer nanocomposite membrane in water treatment. It is very necessary to sort and compare the current complexities and controversies in this field. The further development of polymer nanomaterial composite membranes in water treatment is also pointed out and discussed.