The invention discloses a low pressure upsetting bulge forming method for a hollow component with a complex section to solve the problems that inner high pressure forming is limited by an ultrahigh pressure generating system, thus, the production efficiency is low, and the cost is high. The low pressure upsetting bulge forming method comprises the following steps that a pipe blank of which the section perimeter is larger than the minimum perimeter of the part and not larger than the maximum section perimeter of the part is selected as an initial pipe blank; the initial pipe blank is put into a mold D, and a cavity is filled with a fluid medium and sealed; when a mold C moves to the mold D till the mold C makes contact with the initial pipe blank, the inner pressure of the initial pipe blank is adjusted to be p1; when the mold C continuously moves to the mold D till the mold attaching degree of the initial pipe blank at the section A reaches 90%-100%, the inner pressure of the initial pipe blank is adjusted to be p2; the mold C continuously moves to the mold D till complete mold attaching of the initial pipe blank at the section B is achieved; sealing punching heads and the mold C recede, the part is taken out, and the inner force transferring medium is cleaned; and forming is finished. The technology has the advantages of being high in production efficiency and low in cost.