Based on flood origin and influence mechanism in the Poyanghu Lake, a flood stage calculating model is established, including some variable factors as reduction of lake inflow process, volume variation of lake basin and lake outflow variation. By use of this model, the maximum flood stages in the background years of various decades (1950s, 1960s, 1970s, 1980s and 1990s) from 1952 to 2001 are calculated, and then frequency analysis of the maximum flood stage series is made to obtain 5 groups of flood stage-frequency characteristic values for comparison, so as to find out regularity of lake stage frequency variation. In addition, this paper studies the reason of the flood stage frequency gradually rising in the Poyanghu Lake and estimates the influence of return-land-to-lake on the flood stage frequency.