Seepage tests were performed for the sand sample from typical strata beneath Yangtze River dyke, with grit gravels and a polyaminoester as filter materials, and with dextrose liquor to simulate deoxidization in the stratum. On the basis of the test results and site investigation, the chemical clogging mechanism was analyzed. Ferric mineral in the stratum is deoxidized. While the relief well works as an artesian well in flood season, ferrous ions would be transported toward the well structure and are oxidized to ferric compound at outlet or near the water surface. The compound would be absorbed by the well wall or suspend in the water. While the well drainage stops, oxidization would speed up in the well, ferric compound would moves into the well structure and deposits inside it. So the well would be clogged gradually. This process that the ferrous ions come from the stratum, change to ferric compound and then move back to well structure reveals one of the characteristics of chemical clogging, namely, tracing to source.