The near-trapping phenomenon, which can lead to high wave elevations and large wave drift forces, is investigated by a floating four-column structure. To solve this wave-structure interaction problem, a numerical model is established by combining the wave interaction theory with a higher-order boundary element method. Based on this numerical model, behaviors of scattered waves at near-trapping conditions are studied; and the superposition principle of free-surface waves is introduced to understand this near-trapping phenomenon. To avoid the near-trapping phenomenon and protect the structure, a way for rotating the structure to change the wave-approach angle is adopted, and improvements of the wave elevations around the structure and the wave drift forces acting on each column are found. Moreover, a genetic-algorithm-based optimization method is adopted in order to minimize the total wave drift force acting on the whole structure at various wavenumbers by controlling the draft of floating bodies, the wave-approach angle and the separation distance between adjacent floating bodies. With the final optimized parameters, the wave drift forces both on each column and on the whole structure can be significantly reduced. The optimized arrangement obtained from a certain wavenumber can work not only at this target wavenumber but also at a range of wavenumbers.