The invention discloses a method for performing fast in-situ conversion on a microfiltration or ultrafiltration membrane into a nanofiltration membrane, and belongs to the technical field of membrane separation. By aiming at overcoming the defects of the existing nanofiltration membrane preparation technology, the invention provides a simple, cheap and fast nanofiltration membrane in-situ preparation method. Diamine capable of taking a reaction with graphene oxide without special conditions is used as a cross-linking agent; the fitting performance of a graphene oxide sheet layer is regulated through the combined effect of water surface tension and gas pressure; specific treatment such as heating and drying is not needed in the preparation process; the in-situ preparation of the graphene oxide nanofiltration membrane is realized in a device filled with the microfiltration or ultrafiltration membrane in a simple and fast mode. The method has the advantages that the cost is low; the in-situ preparation can be realized; the regeneration is realized; the limitation by basement membrane types and shapes is avoided. The prepared nanofiltration membrane has high water flux, has a good removal effect on ions and organic matters, and has wide large-scale application prospects.