Diet composition of Whooper Swan (Cygnus cygnus) in winter was studied by the method of fecal microscopic analysis from December 2005 to March 2006.Results showed that Whooper Swan fed on mainly wheat,kelp and eelgrass.Wheat is accounted for 99.67% of food items in winter and 90.17% in early spring.By analyzing the diet differences between winter and early spring and with the results of previous studies,we found that the main original food item eelgrass had been substituted by wheat.This result implied that environmental pollution and wetland destruction might be responsible for the rapid decline of originam food supply to Whooper Swan.