The invention provides a preparation method of a composite basal membrane nanofiltration membrane. the prepration method compriss the following steps: S10, forming step of a composite base-membrane: a non-woven fabric supporting layer is knife-coated with a polysulfone layer by a continuous knife-coating mode; the polysulfone layer is knife-coated with a sulfonated polysulfone layer; the non-woven fabric supporting layer with the polysulfone and sulfonated polysulfone feed liquor enters a gel tank, and polysulfone and sulfonated polysulfone feed liquor undergo phase separation so as to form the composite base-membrane; and S20, interfacial polymerization reaction: 2.1) the composite base-membrane is immersed in a water phase for 2-30 s and then taken out of the water phase, and residual solution on the surface of the composite base-membrane is removed; 2.2) the composite base-membrane is immersed in an oil phase for 2-30 s and then taken out of the oil phase, and a polyamide desalination layer is formed on the surface of the polysulfone-sulfonated polysulfone composite base-membrane; and 2.3) the membrane is further immersed in 5-30 wt% of a pore-keeping solution for 30s-30min and then is taken out and dried so as to prepare the nanofiltration membrane. The prepared nanofiltration membrane has good desalinization rate and water flux level, and service life of the nanofiltration membrane is prolonged due to the composite base-membrane.