A method for forming a plastic support shell of a sole includes the steps of placing a sheet material into a first mold assembly; softening the sheet material; vacuum plastic molding; putting the plastic semi product into a cutting mold assembly; cutting the waste edge; putting into a trimming mold assembly; and trimming with a mechanical arm. The carrying mold and the pressing mold press against the inner and outer surfaces of the plastic semi product, and the restricting edge and the support edge are used to define a cutting line. Therefore, the plastic semi product can be firmly positioned and held tight, and then the mechanical arm can trim the plastic semi product under the guiding of the restricting edge and the support edge, which makes the trimming process easy and precise.