Stable methyl naltrexone injection and preparation method thereof

The invention relates to a stable methyl naltrexone injection or a pharmaceutical salt injection thereof and a preparation method thereof. The injection comprises methyl naltrexone or pharmaceutical salt thereof and additive, wherein the additive is citric acid buffer salt and antioxidant, and the antioxidant is one or more chosen from thiourea, cysteine, L-cysteine, valine, L-valine and nicotinamide. The prescription of the methyl naltrexone injection of the invention uses no EDTA or derivative thereof in the prescription of a foreign market methyl naltrexone injection but has higher stability than the foreign market methyl naltrexone injection. The preparation method of the stable methyl naltrexone injection of the invention has simple process and is applied to mass production.