It caused great influence of uneven settlement on flood discharge foundation from two plastic concrete impervious walls of first-stage cofferdam of No.5 to No.8 flood discharge in Shawan hydropower station. In accordance with plastic concrete impervious walls under flood discharge foundation, this paper calculated settlement and stress distribution of 6 processing schemes in finite element method under different conditions. Numerical calculation research shows that loose blasting should be carried out to plastic concrete impervious wall under 8m of foundation surface, and original sandy gravel should be cleaned and filled the residual impervious wall under 2m of foundation surface. The subsequent operation confirms foundation treatment is correct, which can be used as a reference for similar projects.