By use of the measured water level data, this paper analyzes the varying tendency of the water level of the Poyang Lake in recent 40 years, and describes it quantitatively with the tendency coefficient modulus. It has been shown by the report that the maximum water level is rising at rate of 0.22 m/10 year and its present time is delaying at rate of 3.2 day/10year, but the present frequence of heavy floods is increasing at rate of 0.06 year/10 year. The rise of flood water levels and the increasement of heavy floods cause heavier flood damages. It has become the most important problem during the management and development of the Poyang Lake.