The cast-in-situ concrete thin-wall pipe pile (PCC pile) is a kind of rigid pile suitable for reinforcing soft foundation. A new method of calculating the pile-soil stress ratio of PCC pile composite foundation is presented taking the combined action of pile-soil-cushion into account. The relationship of frictional resistance and relative displacement between the pile shaft and the surrounding soil is idealized as perfectly elastoplasticity; meanwhile, settlement of soil between the piles and inside the piles at the same depth is assumed to be equal, and the soil at the pile tip is considered to be in conformity with Winkler ground model. Based on these assumptions, the calculation formulas for pile-soil stress ratio and settlement are derived through analyzing the pile-soil-cushion interaction. The influencing factors of pile-soil stress ratio are discussed according to the obtained formulas, which are beneficial to the optimal design for PCC pile composite foundation.