The invention relates to a machining method for parts, in particular to a machining method for a casing casting part. According to the technical scheme, the machining method for the casing casting part includes the following steps that firstly, the machining benchmark is cast, wherein the three positioning faces and positioning holes are precisely cast in the casing casting part; secondly, a machining clamp is manufactured, wherein the clamp comprises a disc, jacking blocks, positioning pins and pressing clamping flanges; and thirdly, the casing casting part is machined, wherein the clamp is arranged on a machine tool, the disc is aligned, the casing casting part is arranged on the clamp, and machining is conducted. By means of the machining method for the casing casting part, the machining benchmark is optimized, the brand-new machining strategy is adopted, machining efficiency and the quality of parts are improved effectively, cost is lowered, and the working intensity of labors is lowered.