This paper studies the boundary conditions of interaction between the polar and the soil in sandy foundation soil, defined by the vertical beam on elastic foundation method and the imaginary fixing-points method. Based on a port project, using the structure analysis software SAP2000, we calculate, compare and analyze each pile’s ultimate resistance of the pile and settlement of wharf transverse bent to simulate pile-soil interaction. The result shows that in the sandy foundation soil (when piles are friction piles), the situation which adopts the results by the imaginary fixing-points method as the design basis can be very risky. Therefore, it is suggested to adopt the method of vertical beam on elastic foundation, which is much more reliable and stable.