The invention relates to a sodium demethylcantharidate injection, which comprises sodium demethylcantharidate, cysteine hydrochloride, tertiary butyl p-hydroxy anisole and propylene glycol surfactants in effective curative dose. In the composition, the stability of products is improved greatly by adopting a special process and taking the cysteine hydrochloride and the tertiary butyl p-hydroxy anisole as a combined antioxidant. The invention also discloses a method for preparing the medicinal composition.