The joint solution method of Boussinesq and Geddes solution is used to obtain the distribution of additional stresses at any depth in the composite foundation considering the pile top and the soil top load between piles in the composite foundation. And according to the settlement relationship of flexible pile, interpile soil and composite foundation. The effective pile length calculation method of flexible pile composite foundation with cushion is obtained. The comparison of test and calculation results shows that the effective pile length calculation results are consistent with the actual situation of the composite foundation due to the influence of soil top pressure between piles and the deformation of pile penetration amount on the effective pile length of the composite foundation.