Abstract: Negative skin friction is one of important problems in design of pile foundation,but the negative skin friction of pile groups considering time effect is less studied.The negative skin friction of 3 × 3 pile groups(3d,4d,6d) on the viscosous soil layer under surface load consolidation conditions is studied by model tests.The negative skin friction of pile side,tip resistance of pile and variation of settlement of each soil layer with consolidated time are measured.The position of neutral point and dragload are analyzed.For comparative analysis,a single pile and 2 × 2 pile groups(4d) are tested under the same conditions.The results show that the dragload and position of neutral point caused by the negative skin friction of pile side exhibit significant time effect and pile group effect for both the single pile and pile groups,and they are affected by the number of piles,the position of piles in group and the spacing of piles.It is also shown that the negative skin friction of pile side reaches 80%～ 90 % of the maximum one when the relative displacement of pile-soil reaches 2 mm under the present model test conditions.