Experiments on potherb mustard planted on floatingbed were performed  to study the growth characteristic of potherb mustard and its capacity of absorbing Nitrogen and Phosphorus from eutrophic water body. Results showed that the potherb mustard on floating-bed had a fresh weight of 30 g, culm height of 15cm and root length of 35 cm on average per plant. The amount of directly absorbed Nitrogen and Phosphorus amounted to 67.3 kg/hm² and 4.8 kg/hm² respectively. The results suggested that potherb mustard could grow well throughout the winter and was effective  for eutrophic water purification. It is a new and alternative approach  for the treatment of eutrophic water body in winter.