Based on source code of Environmental Fluid Dynamics Model (EFDC), the water age model and conservative dye model were set up to study water age of seasons and individual rivers of Lake Poyang under natural conditions. Water age calculations indicate that seasonal variations of water age in Lake Poyang are significant. The water age of summer or autumn is smaller, which is increased during reversing process in flood season in most of consecutive years. The water age of winter or spring is larger without occurrence of reversing process. The water age of individual river in five river systems indicates that the exchange of water in the southwest lake zone is mainly influenced by Ganjiang River, whereas in the northwest zone influenced by Ganjiang River and Xinjiang River, in the southern lake zone by Ganjiang River and Xinjiang River, in the eastern lake zone by Raohe River, respectively. The central zone and channel to Yangtze River are influenced by the five rivers together. The proposed Lake Poyang Complex will be rational with principles "water level control during dry seasons not during flood seasons", and the model is proved of reference value for feasibility study of Lake Poyang Complex.