A casting method for a large thin-wall valve body steel casting comprises the following steps: 1, manufacturing a composite physical model: manufacturing an upper half valve body external mold, a lower half valve body external mold, the overall frame of a second core box, and an irregular-shaped section template; filling a mixed filler between the frame and the section template; tamping, smoothly trimming the filler; performing natural curing after the dimensional requirement is met; 2, sand mulling; 3, modeling and core manufacturing; 4, painting, mounting the core, and combining boxes; 5, smelting and casting; 6, performing post-treatment. The large thin-wall valve body steel casting is prepared through the steps. The casting method adopting the composite model can improve the model manufacturing efficiency, saves wood, and reduces the model manufacturing cost; through the adoption of stepped flat ingate system, as well as the manner that a chiller, a riser and compensation are combined, the thin-wall valve body steel casting is protected from fracture, insufficient casting, cold shut, wrinkling, and other casting defects, so as to realize the sequential solidification of the valve body steel casting, and meet the technical requirement of the valve body steel casting.