While nanofiltration (NF) membranes have much potential for precise molecule/ion separations, the thick nanofilms fabricated via rapid and irreversible interfacial polymerization (IP) result in low water permeance. The distribution and location of aqueous monomers play a pivotal role in the IP reaction and thus in the polyamide films, yet are rarely reported to improve the performance. In this study, a one-step spin-coating procedure was utilized to deposit diamines atop porous polysulfone substrates followed by IP reaction for construction of ultrathin polyamide membranes. Direct spin-coating of aqueous diamines onto a porous substrate allows for a uniform diamine deposition, demonstrated by confocal laser scanning microscope analysis. In addition to the uniform distribution, this procedure is able to enrich the diamines beyond the surface pores, contributing to the formation of ultrathin polyamide layers based on Freger’s theory. Importantly, rational regulation of spinning speed, aqueous phase volume, and IP reaction parameters allows to optimize the membrane microstructure and separation performance. The optimal TFC membranes with 9.7-nm-thick polyamide nanofilm evince a remarkably high water permeance of 36.1 L m^-2 h⁻¹ bar⁻¹, and an acceptable divalent Na₂SO₄ rejection of 96.2%. An extended use of spin-coating assisted IP onto other substrates also yields high-flux polyamide NF membranes (TFC-Kevlar: 37.9 L m^-2 h⁻¹ bar⁻¹, 97.5%, TFC-polyacrylonitrile: 41.7 L m^-2 h⁻¹ bar⁻¹, 96.2%), confirming the applicability and high efficiency of this approach. In contrast with established methods, spin-coating assisted IP (SCIP) features distinct superiorities in economizing diamine dosage, ease of control, and attaining an excellent sieving performance, which paves the way of constructing high-performance NF membranes.