Abstract: The predecessors have been investigating on the interaction between pile and soil of box-foundation with friction pile-group under highrise buildings for a long time. The results of these investigation give rise to much controversy, and can not be used to solve the practical problem in engineering design. In this paper, the mechanism of interaction between pile and soil is fully brought to light, based on the prototype observation of one highrise building and the model test in laboratory. The results of the tests demonstrate clearly that the pile and soil work to-gether to bear the load from the superstructure. Under given conditions, the piles bear 80% of the load, and the soil between the piles bear 20% of the load. The load distribution curve during the variation of load from the allowable load to the ultimate load and the failure load has been given, which provides the theoretical basis for rational design of bos-foundation with friction pile-group.