Calculation of negative skin friction on piles in areas of soft soil and collapsible loess is one of the issues must be considered in pile foundation design and construction; while studies on the calculation of negative skin friction on pile groups are relatively little at present. Effective stress formula

negative skin friction on single pile is calculated. Based on limit equilibrium theory

the calculation method of the pile group effect coefficient under negative skin friction is obtained. With field test results

negative skin friction and drag-load on X-section cast-in-place pile groups are analyzed. The influence of location of piles in pile foundation on drag-load along pile shaft is discussed; and the change rule of the group effect coefficient varies with different influencing factors. The results of calculation and field measured results are in good agreement; the greater the surface load is

the greater the drag-load on pile groups is

and the pile group effect coefficient increases with pile spacing increase and it decreases with the increase of negative skin friction coefficient and the depth ratio of neutral point.