In order to test ecological effects of Cyanobacteria lysed by Cyanophage on the nutrition cycle of fresh water, an experiment was carried out in six cement ponds with culture of the Plectonema boryanum IU594. Of these ponds, three were inoculated with the Cyanophage PP at 106 PFU•ml^－1 and added the duckweed. The other three ponds were inoculated with the Cyanophage PP only as control. After the treatment, the concentration of chlorophyll a, ammonia, nitrate, phosphate, total phosphorous, total nitrogen, the number of bacteria and the fresh weight of duckweed were monitored. It was found that lysis of Cyanobacteria by Cyanophage caused rapid release of the nutritional elements and brought the bacteria into a new life cycle, and led a new bloom in the control. But the duckweed could absorb the nutrition elements released from lysed cells and exhibited inhibition of the new bloom from the new life cycle in experimental group. It may be inferred that floating plants such as duckweed accompanying algal-lysing microorganisms e.g. Cyanophage could eliminate the new algal bloom and improve water quality.