With the construction of deep-water bridge foundation, steel casing is left after the construction of cast-in place piles due to the dismantle difficulty. Then the pile actually forms a variable section pile which has bigger section area at upper part. Using the nonlinear NL method, the displacement and the internal force are analyzed for piles with and without casing in Sutong Bridge. It is found that the displacement of pile with casing can be reduced by 50%, and it can not be ignored for accurate calculation of displacement. 8 model pile tests are carried out, which suggests that the slurry thickness can significantly affect the capability of co-working between the pile and the casing.