According to the situation of pile hole diameter reducing and borehole detection data, and based on the engineering investigation report parameters, the causes of hole diameter reducing in thick dredger fill formation in coastal region and the corresponding relation between time and volume are analyzed. The solving methods were put forward, one is by adding bentonite or additive to increase mud weight and adjust other technical indexes in order to improve the performance of wall consolidation; another is by diameter expanding. And accurate control of the expanding amount is particularly important. With the hole formation test and construction data, the corresponding relation between diameter reducing amount and time is analyzed to find out the diameter reducing rules in order to obtain correct diameter expanding amount to provide the reference for the construction.