The invention belongs to the technical field of megawatt nuclear power station auxiliary cooling water and discloses a megawatt nuclear power plant spent fuel pool expansion method. The expansion method comprises the following steps: dismantling an old grillwork: lifting the old grillwork placed in a spent fuel pool by using a first lifting device, and conveying to an appointed area by using transportation equipment for storage; leveling a high-density grillwork: leveling the high-density grillwork by a leveling device to adapt to the height of the pool bottom of the spent fuel pool; mountingthe high-density grillwork: placing the high-density grillwork at the position of the old grillwork in the spent fuel pool by using a second lifting device; transferring spent fuel: transferring the spent fuel into the high-density grillwork for storage. Through the expansion method provided by the invention, the problems that the spent fuel pool is full and the storage requirement of the spent fuel in the nuclear power plant cannot be met are solved, so that possibility is provided for subsequent and continuous storage of the spent fuel.