Abstract: The unsteady ground effect of a multi-element airfoil during its taking off and landing has been studied. For the airfoil's large displacement, a strategy that mesh around airfoil should be kept in rigid motion with the airfoil is proposed. Besides, a simple and practical mesh reconstruction method based on moving mesh has been adopted to preserve the grid quality. Furthermore, the Navier-Stokes equations in arbitrary Lagrangian-Eulerian (ALE) frame have been solved to simulate the unsteady flows around the airfoil close to the ground. Both NACA0012 and GAW-(1) two-element airfoil in uniform down have been computed out of ground effect, and the results are in agreement with calculations of the corresponding quasi-steady cases. As GAW-(1) two-element airfoil approaches the ground, the unsteady ground effect has been investigated. Computations indicate that the lift of the multielement airfoil decreases as it gets close to the ground. Besides, the results are compared with the quasi-steady (add the equivalent attack angle to airfoil's attack angle) ones, which show that unsteady ground effect is first less then greater than the quasi-steady one as the height between airfoil and ground decreases.