Provided is a method for producing a welded steel pipe having excellent buckling resistance. The method for producing a welded steel pipe according to the present embodiment includes steps of: preparing a welded pipe, and expanding the welded pipe over the entire length thereof by using a pipe expanding head such that an undulation wavelength ratio D defined by Formula (1) is not more than 0.8, or not less than 1.8: D=p/λ  (1), where p is an undulation wavelength in the axial direction of the welded steel pipe, and λ is a Timoshenko's buckling wavelength as defined by Formula (2): λ=3.44×(r×t)1/2   (2), where r is an inner radius of the welded steel pipe and t is a wall thickness of the welded steel pipe.