A method of preparing a thermoplastic composition is provided. The method includes the following steps. A polyetherimide or a polyphenylene sulfide is provided. A polyimide is provided, wherein the glass transition temperature of the polyimide is between 128° C. and 169° C., the 10% thermogravimetric loss temperature of the polyimide is between 490° C. and 534° C., and when the polyimide is dissolved in N-methyl-2-pyrrolidone and the solid content of the polyimide is 30 wt %, the viscosity of the polyimide is between 100 cP and 250 cP. A melt process is performed to mix the polyetherimide and the polyimide or mix the polyphenylene sulfide and the polyimide to form a thermoplastic composition. Further, a thermoplastic composition is also provided.