In this paper, a modified PZIII model by considering the impact of principal stress rotation for the wave(current)-induced soil response in a sandy seabed is cited. Unlike the previous works, the proposed model considers the effect of PSR by treating it generating the plastic strain rate independently. Then, the proposed model was incorporated into the finite element analysis procedure DIANA-SWANDYNE II. Both wave and current loadings are considered in the present model. The proposed model was first validated through comparisons with the previous experimental data for the soil response under wave and current loading. Adapting the proposed model, effects of the PSR on the fluid-seabed interactions will be investigated.