The spacing between piles is directly related to the incident wave force on piles and the drag force on the platform or the upper structure.A three-dimensional numerical wave flume was developed based on software FLOW-3D,and an experimental investigation was carried out firstly to confirm the efficiency and accuracy of the numerical simulation,and then the influence of spacing L on the wave force on both upstream and downstream piles was numerically studied.The results indicate that both forces on upstream plies parallel to the wave direction and perpendicular to the wave direction are different from the case of isolated pile because of the interaction of the piles,and the grouping effect coefficient KG is closely related to the spacing L.For grouping effect coefficient parallel to the wave direction,when the two plies are in small spacing,the upstream plie KGxu＜1.0,and the downstream plie KGxd＞1.0,and the grouping effect coefficient KGy perpendicular to the wave direction can reach 3.0～4.0.With the increase in the spacing between the piles,KGxu increases gradually,whereas KGxu and KGy decrease.The interaction of piles can be neglected when the ratio of spacing L to the pile diameter D reaches 4.0.