The embodiment of the invention discloses a method for welding a piston by an electron beam, which comprises the following steps: respectively machining the head and skirt of a piston; heating the head of the piston in a heating furnace; assembling the head and the skirt of the piston; putting the integral piston in the heating furnace to weld a circumferential seam between the head and the skirtof piston; carrying out thermal insulation on the welded piston; putting the integral piston in the heating furnace to weld the top seam of the piston; carrying out thermal insulation on the welded piston; and carrying out heat treatment on the piston, and machining the integral piston. The method ensures that 99% of air holes (3*5 mm) at the welding arc-off position of the welded piston can not be more than 10, thereby greatly enhancing the performance of the welded piston.