Dam stability has always been a challenge for the flood control, and high flood water level in flood season and steep drop of water level in flood recession often lead to seepage failure, instability of inner and outer slopes, and even dam breaches. The physical and mechanical parameters of the rock and soil of a damaged dam in flood season were obtained by the geological method, and the dam stability in flood season was inversed by the finite element method modeling. With the measured data of seepage and deformation instability of the dam in different water levels in flood season, the parameters used in the simulation calculation were calibrated, and the seepage and anti-sliding stability of the designed reinforced dam were calculated and analyzed. The results indicate that the seepage failure and slope instability will occur when the original dam is at designed water level, dam crest water level and steeply dropped water level before flood season. Whereas the inner and outer slopes of the reinforced dam will maintain stable when the reinforced dam is at dam crest water level and steeply dropped water level, but seepage failure will occur at the inner and outer dam feet at dam crest water level and designed water level. After adjusting the reinforcement scheme, the anti-seepage and anti-sliding stability can meet the requirements; therefore the adjusted design scheme is feasible.