The invention relates to a plastic composition for a cap. The plastic composition consists of the following materials in percentage by weight: 30-50 percent of bio-based plastic and 70-50 percent of polypropylene plastic, preferably 30 percent of bio-based plastic and 70 percent of polypropylene plastic, or 40 percent of bio-based plastic and 60 percent of polypropylene plastic, or 50 percent of bio-based plastic and 50 percent of polypropylene plastic, wherein the bio-based plastic is preferably starch-based plastic. The invention also relates to a processing method for producing a cap, wherein the cap is made of the plastic composition for the cap through injection molding; and processing temperature of the injection molding is between 180 DEG C and 205 DEG C. The invention also relates to a cap which is manufactured by using the processing method for producing the cap. The plastic composition for the cap has unique and skilful conception; consumption of petrochemical resources and emission of carbon dioxide can be reduced; the plastic composition is energy-saving and environmentally-friendly; service performance of the manufactured cap is guaranteed; and the plastic composition is suitable for large-scale popularization and application.