The content of total organic carbon (TOC), total nitrogen (TN), the C/N ratio and the carbon and nitrogen stable isotope compositions of the sedimentary organic matter, fish feed and fish feces were measured from the cage fish farm in Lake Poyang. The origins of the sedimentary organic matter were analyzed using the carbon and nitrogen isotope compositions and C/N ratios. The results showed that the δ¹³C and δ¹⁵N values of sediments ranged from -27.67‰ to -25.65‰ and from 5.19‰ to 7.27‰, respectively. The δ¹³C values of waste feed and fish feces were -24.73‰ and -26.30‰, respectively. And the δ¹⁵N values of waste feed and fish feces were 10.28‰ and 15.54‰, respectively. The sedimentary organic matter of the cage fish farm had a mixed source of waste feed, plankton, and the other organic matter. The mean proportions of the waste feed, plankton, and the other organic matter were 48.3%±11.4%, 25.6%±11.3%, and 26.0%±5.8%. The contribution of fish feces to the sedimentary organic matter pool was insignificant. The waste feed seemed to disperse about 1500 m distance from the cage under the influence of the dispersion and resuspension. During the period of fry, the waste feed of cage fish culture was the main source of the sedimentary organic matter in the cage fish farm area of Lake Poyang.