Novel crystalline form of perphenazine 4-aminobutyrate trimesylate and a process of producing the same are disclosed. The novel crystalline form is characterized by a unique XRPD pattern and a DSC that exhibits an endothermic peak at a relatively high temperature (e.g., higher than 209° C.). Also disclosed are a process of preparing perphenazine 4-aminobutyrate trimesylate by in situ deprotection and salification, in a single-step synthesis, and a highly pure perphenazine 4-aminobutyrate trimesylate obtained thereby. Uses of any of the described perphenazine 4-aminobutyrate trimesylate are also disclosed.