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 preparing 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.