Abstract: Large-scale amplitude liquid sloshing in a rectangularopen tank under a pitching excitation is studied in this paper. Firstly, thekinematics of the ALE (arbitrary Lagrange-Euler) description isintroduced and the Navier-Stokesequations of fluid are revised in the ALE form. The numerical discretedequations of fractional step finite element method are developed byusing Galerkin weighted residual method. The numerical simulation of largeamplitude sloshing of the liquid in a rectangular tank is carried out. Thecomputed water elevation history is obtained. Through comparing thenumerical results with experiments, the efficiency of fractional step ALEfinite element method is confirmed.