The invention relates to a charged nanofiltration membrane and a preparation method thereof. The method comprises the steps of preparing charged polymer solution, swelling, de-swelling and the like. The aperture size of the original hollow fiber ultrafiltration membrane is directly reduced to nanofiltration level of aperture size, the structure of the membrane is not damaged, and the charged nanofiltration membrane is prepared with a one-step method. Moreover, according to the method, operation conditions are mild, a special chemical reagent is not needed, and equipment requirement and preparation cost are extremely low, so that the charged nanofiltration membrane is extremely suitable for large-scale industrial production.