The invention relates to a machining method of a seal head of a nuclear power water chamber. The method comprises steps as follows: a spherical segment blank is obtained through die forging; four auxiliary press blocks are uniformly distributed and welded on the outer side of the spherical segment surface of the spherical segment blank and are compressed through external pressing plates, a support sleeve is processed, the upper end surface of the support sleeve is arc-shaped and is matched with the lower end of the spherical segment blank, and then the spherical segment blank is positioned and mounted on the support sleeve; the support sleeve is used for positioning a nuclear power water chamber seal head assembly, the upper end surface of an opening of the spherical segment blank is turned to be flat, and internal and external welding grooves are turned in the upper end surface of the opening; the external bottom surface of the spherical segment blank is positioned on a machine tool table for cutting and machining an inner hole and a welding groove; the four auxiliary press blocks are removed, the surface is smoothened, figure numbers are printed, and machining of the nuclear power water chamber seal head is finished. With the adoption of the support sleeve, the problem about positioning during turning of the nuclear power water chamber seal head assembly is solved; with the adoption of the technological scheme of welding the auxiliary press blocks, the clamping firmness is greatly increased, and the turning machining efficiency is greatly improved during turning of the bottom surface.