The invention discloses a forging method of a lower sealing head forge piece of a one-mega kilowatt nuclear-power reactor pressure vessel. In the method, a double-vacuum steel ingot made of 16MND5 with a weight of 103 tons is forged by using a hydraulic press of 12,000 tons, wherein the forging process sequentially comprises the following steps of: drawing: heating the double-vacuum steel ingot to 1,220+ / -10 DEG C, and changing a length of 2,625mm into 4,600mm; gas cutting blanking: removing a section of the bottom end of the forge piece to change the length of 4,600mm into 3,450mm, wherein the bottom of the double-vacuum steel ingot is guaranteed to be sufficiently cut off; upsetting: raising the temperature to 1,240+ / -10 DEG C and changing the length of 3,450mm into 290mm; rough machining: roughly machining the forge piece blank into a cake shape, wherein the diameter of the cake is 5,000mm, and the thickness is 220mm; and stamping and forming: heating the forge piece to 1,000+ / -10 DEG C and putting the forge piece on a special die for stamping and forming until the forging ratio reaches 1.2, and the forge piece forms a spherical crown shape. The invention can ensure that the forged lower sealing forge piece has the advantages of dense material, uniform component, reasonable metal streamline distribution and stable property.