The hydrodynamic conditions near a floating structure are complex during its operation in the flowing water. The floating structure is easy to overturn if it gets improper handling. The overturning moment is an important factor that affects the stability of floating structure. Based on a CFD method, the overturning moment and its variation rules with different length-height ratios, different heights from the bottom and different water heads in the flowing water were explored in this paper using an appropriate grid scale. The results show that the overturning moment of floating structure decreases after the first increasing with the growth of height from the bottom; also, the overturning moment of floating structure has a tendency of growth with the increase of length-height ratio and the water head. This study can offer reference for the understanding of overturning mechanism and stability design of floating structure in flowing water.