A vertical loading test is carried out on one intact pile and eight necking piles through transparent soil technology，and the vertical capacity of the necking pile is studied. The speckle field of soil around the pile is processed using MatPIV software to obtain the soil displacement law. On this basis，the reasons for the change of ver? tical bearing capacity of piles are analyzed compared with the intact pile. The results show that the necking has a sig? nificant contribution to pile capacity. When the necking length is 10% of the pile length，the bearing capacity loss can be as high as 45%；When the necking diameter is 20%，the bearing capacity loss is 27.3%. The reason is that when the necking dimension is large，the soil around the necking develops inclined and downward displacement， causing the relative displacement of the pile shaft and the soil to reduce，and reducing the side shaft friction of the pile. Consequently，the necking makes the pile capacity reduce. For some necking piles，the pile capacity is insensitive to the necking because of the combined action of the resistance generated by the necking，the resistance at the pile end，and the shaft friction. However，its large settlement leads to the result that it is unsuitable for bearing. When the necking locates in the shallow or middle parts of pile shaft，the pile capacity is seriously lost. When the necking locates in the deep parts of the pile shaft，the soil at the necking position and at the end of the pile runs through each other，and its shear failure occurs. The research achievement can provide theoretical and technical ref? erences for the design and reinforcement of pile foundations.