Abstract: The present invention has provided a welding forged steel single piece piston and its manufacturing methods. A semi-finished head and a semi-finished skirt are forged respectively; the semi-finished head and semi-finished skirt are heat-treated respectively; rough machining is carried out respectively to the semi-finished head and the semi-finished skirt, and a face of weld between the two adjacent ring grooves is higher than the one under the combustion chamber; the head and the skirt are welded together through friction welding; an annular groove, a combustion chamber, a ring groove, a pinhole, an outer annulus and oil inlet/outlet holes are manufactured respectively; and the surface of the piston is phosphorized and graphitized. Compared with one-piece forged piston, it has better processing technology, because the head and skirt are forged respectively and then welded together.