Sinking of cement mixing piles has occurred in the treatment process of the soft soil foundation of an expressway in Jiangsu Province. Based on field data of the excess pore water pressure, earth pressure, and shear strength of the soil around piles, the mechanisms of the sinking of the mixing piles were analyzed, and methods for identifying the sinking are proposed. The field survey results show that the excess pore water pressure was larger than the earth pressure, and a phenomenon similar to liquefaction occurred in the soil around the piles when sinking occurred. This can be used as the identification criterion of the sinking of mixing piles.