The invention relates to a piston assembly of a single welding machine. The piston assembly comprises a machine seat, wherein a connection rod friction sheet assembly is arranged in the machine seat; the upper part of the connection rod friction sheet assembly is arranged in the lower part of a piston in a sleeving manner; sealing rings are arranged at the outer circumference of the middle part of the piston; the upper part of the piston is arranged in a machine seat cover; the machine seat cover is connected with the machine seat; closed spaces are formed in parts, which are positioned on two sides of the piston, of the machine seat cover. The piston assembly of the single welding machine has the advantages that the surface areas of the closed spaces under the gas action are reduced, and the downward acting force of the piston and the connection rod friction sheet assembly is reduced.