Based on the layer by layer (LbL) assembly technology, the nano-scale composite membrane with excellent structure can be prepared by changing the polyelectrolyte and controlling the deposition conditions. Polyamines and phosphate ions (Pi) can be self-organized to form supramolecular systems which could be exploited to stabilize the interfacial architecture. The LbL membrane was made of the positively charged poly (allylamine hydrochloride) (PAH) self-organized with Pi and the negatively charged poly (sodium 4-styrene sulfonate) (PSS) alternatively on top of a poly (acrylonitrile) (PAN) substrate. Compared to the membrane without Pi, the membrane assembled by PAH/Pi improved forward of flux and reduced flux of salt. The concentration of PAH and PSS, deposition time, pH and number of layers showed significant influences on the performance of the membrane. In this experiment, we systematically investigated the preparation conditions and under the optimized conditions the prepared membrane exhibited high water fluxes of 13.5 L m −2 h −1 with corresponding salt to water flux, Js/Jv, ratio of 0.07 g L −1 tested by forward osmosis when DI water as feed solution and 2 mol L −1 MgCl 2 as draw solution.