The present invention relates to a new toxin and a method for preparing an intermediate thereof. In particular, the present invention relates to a new toxin and a method for preparating intermediates (III) and (IV) thereof, and a preparation method for synthesizing what is shown in general formula (I). The method comprises subjecting a chiral compound shown in general formula (III) to a series of protective group addition, protective group removal and amidation so as to obtain a compound as shown in general formula(I). The present method has the advantages of mild reaction condition, simple operation, high optical purity and high synthetic yield, and is suitable for large-scale production.