The operation of the Three Gorges Project(TGP) modified water sand regime in the middle and lower reaches of Yangtze River, affected on water level in Lake Poyang and correspondingly brought series new problems on water resources utilization, water quality, wetland and ecology. Analysis of daily water level turn out:yearly variation of water level in the lake includes four stages-low level stage, level rising stage, backwater stage, and recession stage; water level in lake is determined by Yangtze discharge in backwater stage, and is affected by both Yangtze discharge and discharge at Hukou station in the other three stages. The operation of TGP does not change the basic feature of Lake Poyang-"lake-like with high water level and river-like with low water level", but actually changes values of water level. Physical model experiments have been carried out to study the effect of TGP on water level in Lake Poyang, and the results indicate:TGP impoundment caused decline in water level, the average(maximal) value of decline at Duchang station is 0.94 m(2.58 m), and corresponding decline ratio of water-surface area of Lake Poyang is 68%; TGP pre-discharge caused increase in water level, the maximal value at Duchang station is about 1 m, corresponding increase ratio of water-surface area is 32%; the operation of TGP in dry season affects little on water level in Lake Poyang.