Abstract: In order to reveal the composition variation characteristics of dissolved organic matter(DOM) in the water of the Middle Route of South-to-North Water Diversion Project, we collected water samples in the main channel of the Middle Route of South-to-North Water Diversion Project and Danjiangkou Reservoir, and analyzed the molecular composition and spatial variation of DOM by Fourier Transform Ion Cyclotron Resonance Mass Spectrometry(FT-ICR MS).The causes and environmental significance of DOM changes along the main canal were explored.The results showed that more than 3 200 kinds of DOM substances were detected in the Danjiangkou Reservoir and the main canal.Substances only containing C,H,and O(CHOs) were the main components, accounting for 61% of all detected substances; substances containing C,H,O,and N(CHONs) accounted for 30.5%～31.5%;while substances containing C,H,O,and S(CHOSs) only accounted for 7.3%～8.4%.Based on the O/C and H/C classification, the main component of DOM in both the Danjiangkou Reservoir and the main canal was lignin(over 60%).Compared with the Danjiangkou Reservoir, the composition of DOM in the water of the main canal has undergone significant changes, with the proportion of protein-like substances increasing from 3.70% to 8.65%,indicating the generation of protein in the main canal; the proportion of condensed ring-like substances decreased from 9.43% to 4.77%,and the trend of tannic acid-like substances was similar to that of condensed ring-like substances.The main reason for the increase in the proportion of CHONs and CHOSs-like DOM components in the main canal was the local algae proliferation process.The research results can provide a basis for the scientific understanding of the water quality changes of the Middle Route of the South-to-North Water Diversion Project.