We discriminated the origins of suspended organic matters in different seasons and water levels by carbon and nitrogen stable isotopes (δ¹³C and δ¹⁵N) and ratio (C/N) in Lake Dahuchi and Lake Shahu, national nature reserve of Poyang Lake in Jiangxi Province. The results showed that δ¹³C or δ¹⁵N varied in different water levels. When water level rose in May, the δ¹³C reached the highest value with average -26.4‰±0.9‰. In the dry season October, the δ¹³C has the lowest value, average -31.2‰±1.1‰. The δ¹⁵N were of lower values in flooding seasons, however relativity high values in dry seasons. The C/N in October were lowest, average 6.5±0.6 and less than 7. In other months C/N were between 7.8 and 8.7. The variation of δ¹³C, δ¹⁵N and C/N ratio in each month was similar between Lake Dahuchi and Lake Shahu, without significant difference. However, the δ¹³C and δ¹⁵N in Lake Dahuchi were more than 0.1‰-0.5‰ and C/N were lower than 0-0.4 than those of Lake Shahu. At high water levels in May and August, the suspended organic matters were mainly from soil through rivers transportation, with 16% from surface sediment in May and 25% from aquatic floating-leaved plant in August. The suspended organic matters in October were mainly from algae (77%). At lower water levels of December, March and April, the suspended organic matters were mainly from surface sediments, and the excrement of migratory bird accounted for 40% in December. The phyto-detritus in wetland accounted for 47% and 51% in March and April, respectively.