PURPOSE: A manufacturing method of a thermoplastic resin and the thermoplastic resin manufactured thereby are provided to improve the mechanical property, the weather ability, and the color of the thermoplastic resin. CONSTITUTION: A manufacturing method of a thermoplastic resin comprises the following steps: producing a reaction mixture by mixing a reaction solvent, a styrene-based monomer, an acrylonitrile-based monomer, ethylene propylene rubber, and methacrylic acid; firstly continuously bulk-polymerizing the reaction mixture in 90~130 deg C; and secondly continuously bulk-polymerizing the mixture in 130~160deg C. 5~10wt% of methacrylic acid is contained in the mixture.