The invention relates to the technical field of preparation of water treatment materials, in particular to a method for preparing a polyamide nanofiltration membrane by an interfacial polymerization process. The method comprises the following steps of immersing a polysulfone ultrafiltration membrane in a p-xylylenediamine aqueous solution, and performing soaking for enabling the surface of the polysulfone ultrafiltration membrane to perform saturation adsorption on p-xylylenediamine molecules; removing excess liquid after the polysulfone ultrafiltration membrane is taken out, so that the surface of the PSF ultrafiltration membrane is free of obvious liquid drops; pouring a trimesoyl chloride solution on the surface of the PSF ultrafiltration membrane, and removing an oil phase solution after a reaction to obtain a polyamide nanofiltration membrane; placing the polyamide nanofiltration membrane in an oven with the temperature of 40-90 DEG C for heat treatment to promote a subsequent polymerization reaction; and after the reaction is finished, immersing the nanofiltration membrane in an NaHSO3 solution for later use. The preparation method is simple and mild in condition; and the obtained nanofiltration membrane has excellent nanofiltration performance, the semi-aromatic amide structure of the nanofiltration membrane has good desalination performance and chlorine resistance, andthe requirements of seawater desalination treatment under various conditions can be met.