Water quality was polluted to a certain degree in main stream of Tarim River under the effect of human activities．The results of water quality assessment showed that the pollution of water quality was very light in source flows and serious in the upper and middle reaches of Tarim River. Because water was input from Kongque River in the lower reaches of Tarim River,pollution of water quality in this area was lighter than that in the upper and mid– dle reaches of Tarim River．The degree of mineralization,content of sulphate and chloride a– mong the surveying index are high,they caused pollution of water quality．