Hole check-up is conducted on the construction of drilling cast-in-place piles in a practical engineering of Shanghai. Firstly, the changing law of hole diameter over time and space is summarized by analyzing the data of hole diameter considering unloading effect. The diameter reduction and time have positive correlation, and the reduction range is decreased gradually over time, until the hole diameter is stable. Variable quantity of hole diameter is disparate in different soil layers. The smaller the cohesion c is and the bigger the internal friction angle φ is, the poorer the stability of hole wall will be and the larger the variable quantity of hole diameter is. Secondly, a scientific and reasonable fitting formula is applied to fit the change of hole diameter over time in different soil layers; it is shown that the formula is of high degree of fitting and can truthfully reflect the changing law of hole diameter. At last, based on the Kelvin model, a changing formula of hole diameter is deduced by utilizing elastic theories and combining the fitting formula to explain the case more exactly. And another formula which is concerned with the stability of hole wall is also deduced to explain the different stabilities of different soil layers more reasonably. Some conclusions from this paper are of great reference value in the construction of hole-forming in drilling cast-in-place piles in Shanghai area.