In order to correctly describe the pile-soil interaction in deep soft soil and solve the ultimate resistance of the pile side, the pile-soil interaction of different pile diameters and depths in soft soil are studied by using the finite difference method. Based on the Soft-Soil constitutive model, the stress and deformation of the soil around the pile and the ultimate resistance of the pile side are discussed and analyzed, and the application of empirical formulas such as soft soil modulus, strength parameters and p-y curve in deep soft soil layers is modified and optimized. The research shows that: 1) The lateral load level of the pile has a great influence on the distribution of pile resistance around the pile. Under the action of lateral load, the soil resistance around the pile is distributed in an elliptical manner; 2) The revised formula can better describe the pile-soil interaction in soft soil layers; 3) When the soft soil layer is deep (greater than 25 m), the numerical calculation results are consistent with the calculation results of the modified formula. The results can provide some reference for the design of pile foundation in deep soft soil layers.