Abstract: Four 2-DOF computational models of rigid underwater flapping foil are established to compare the propulsion performance, and to study the influence mechanism on flow fields, respectively. Based on the Realizable k-ε turbulent model, the Unsteady Reynolds Average N-S (URANS) equations are solved by finite volume method (FVM), and the numerical simulations about unsteady hydrodynamic characteristics of four underwater flapping foil motion modes are performed through dynamic mesh technique, and the structure of flow fields and the influence on it are analyzed. Numerical results indicate that:the influence of four flapping modes on the propulsion performance of underwater flapping foil is extremely diverse. The mean thrust corresponding to mode 3 is the largest, and there exists no drag during the whole motion cycle; the mean thrust corresponding to mode 1 is less than the former, and the mean thrusts corresponding to mode 2 and 4 are negative; the mean lifts corresponding to the four modes are all nearly zero, the instantaneous lift peak of mode 1 are the largest, and mode 3 can generate the smallest instantaneous lift peak.