Abstract: By the indoor model test of flyash pile, the deformation and failure characteristics of flyash pile, the change pattern of pile and soil stress and the mechanism of flayash pile to reinforce soft foundation are analysed. Firstly, the soil stress increases gradually with the load increasing, and the pile body stress increases rapidly, the pile will yield and the composite foundation will begin to failure when the pile stress reaches the peak strength, then the stress will decrease after the peak strength; Secondly, the stress ratio of pile to soil changes with the load and time, and it is related to the pile soil modulus ratio and the length diameter ratio of pile. So the pile soil stress ratio should be considered a variable when the pile foundation is designed; Finally, because of the bigger strength and good permeability of flyash pile, its compacting and underpinning functions and time effect are all prominent when soft foundation reinforced. Therefore flyash pile is proved to be a kind of more suitable flexible material pile.