The natural wetland change in Hongze lake over long term has been studied using remote sensing image. The fusion classification method was employed in a combination of extensive field data to classify the ETM+image acquired on August 21, 2002. Then the thematic results were output to feature space of infrared versus red bands and the clusters were overlaid upon the thematic map. Further the clustering was implemented on the MSS data of August 6, 1979. Those clusters were imported into feature space. The clusters were then labeled using a combination of False Color Composites of MSS, the results of investigation, and the relationship between clusters in feature space. Finally, post-classification comparison was employed to quantify the magnitude of the changed areas between the two dates. The results indicated that a loss of 6 458.59 hm2 of natural wetland in the study area occurred, and the loss of natural wetland was the result of unreasonable development of wetland in the watershed.