Konjac glucomannan (KGM) with good water solubility was extracted and purified by a cheap and simple method including fat-extraction, protein-extraction, decolouring, centrifuge, precipitate and other procession from tubers of amorphophallus riveiri. Acrylamide grafted konjac glucomannan (AKGM) was prepared in the presence of Ce(IV) serving as initiator in order to elucidate its molecular weight. Results from the measurements of capillary viscosity by figuring out the specific viscosity and applying to Mark-Howink equation showed that the viscosity-average molecular weight of acrylamide grafted konjac glucomannan was about 9.58×105.