The invention discloses a welding support and method for a horizontal HDPE pipeline of a siphonic type rainwater drainage system and relates to the technical field of construction of the siphonic type rainwater drainage systems. The welding support facilitating welding of the horizontal HDPE pipeline of the siphonic type rainwater drainage system is provided. The welding support comprises an installation piece, two portal frames, a welding platform and two pipe supporting frames, wherein the installation piece is provided with an installation groove and two portal frame grooves, the upper portions of the two portal frames are hung on the installation piece, the two portal frames are located in the two portal frame grooves, the lower portion of each portal frame is provided with two connection screw rods, the four connection screw rods penetrate through the four corners of the welding platform correspondingly, supporting nuts are connected to the connection screw rods, the supporting nuts support the welding platform, the two pipe supporting frames are arranged on the welding platform in a spaced manner, the top of each pipe supporting frame is a V-shaped pipe supporting plate, and the height of the pipe supporting frames can be adjusted. By the adoption of the welding support, the HDPE pipes and a butt welding machine can be supported, and the two HDPE pipes can be conveniently made opposite to each other.