Liquid sloshing in ship tanks would have a direct effect on ship dynamic stability, and thus is of great importance for navigation safety. To calculate the sloshing-induced force on real ship tanks, this paper presents an approach to numerically simulate the liquid-sloshing in complex tanks using the moving particle semi-implicit (MPS) method. The sloshing-induced force is numerically calculated and used to investigate the effect of different excitation conditions in which the realistic ship motions under different loading conditions have been taken into account. Simulation results show that the maximum sloshing-induced force is much bigger than the corresponding static one. Meanwhile, both the rolling angle and period have significant effects on liquid sloshing.