The invention relates to a new process method for manufacturing a mold capable of ensuring the size accuracy of a cast blade for a large-scale water turbine. The method is characterized by comprising the following steps of: selecting materials; designing a technical scheme for manufacturing the mold; establishing a three-dimensional model for manufacturing the mold; manufacturing a mold blank; processing the mold; and detecting the mold. The process method of the invention is unique and solves the problem that a distorted-surface blade casting mold has low manufacturing efficiency and that the kind of casting mold manufactured by the traditional manual method cannot achieve the accuracy of a design size. The method solves the deformation problem of the mold per se and ensures the size accuracy of a blade-type mold through the research and application of a new material and a new method in a mold manufacturing process.