The antitumor activity of konjac glucomannan (KGM) short chain polymer of rhizoma amorphophalli was discussed in this study. The results showed that KGM short chain polymer of rhizoma amorphophalli inhibited the growth of HeLa cells with the unobvious effect. KGM short chain polymer of rhizoma amorphophalli inhibited the tumor growth of S180 tumor bearing mice under different doses. Moreover, the antitumor activity increased with the increasing doses. The molecular structure of rhizoma amorphophalli was analyzed by X-ray diffraction, infrared spectroscopy, Raman spectroscopy, and circular dichroism. The results showed that the antitumor activity of rhizoma amorphophalli was related to the KGM content.