The invention discloses an energy-saving 
nanofiltration membrane for a water purification 
system and a preparation method of the energy-saving 
nanofiltration membrane. The energy-saving 
nanofiltration membrane is composed of non-woven cloth, a 
polysulfone supporting layer, a loose 
polyamide separating layer and a dense positive charge 
thin layer. By properly hydrolyzing the dense nanofiltration membrane, the molecular weight 
cut-off of the nanofiltration membrane can be increased, and part of 
monovalent ions are selectively allowed to permeate while it is guaranteed that small 
organic molecules are effectively intercepted, so that the nanofiltration membrane still has a high 
permeation flux under high-
salinity water inlet or a high 
recovery rate, and it is unnecessary to additionally increase operating pressure. By depositing the dense positive charge 
thin layer on the surface of the loose 
polyamide separating layer through 
electrostatic adsorption-chemical crosslinking, interception of the nanofiltration membrane on multivalent cations (such as 
calcium ions, 
magnesium ions, heavy 
metal and other substances harmful to the 
human body) in water can be easily improved, and drinking 
water safety can be guaranteed to a great degree; meanwhile, the thickness of the deposition 
coating can be effectively controlled within a certain range with the 
electrostatic adsorption technology, and thus substantial increase of 
permeation resistance can be avoided as far as possible.