Abstract: The article states the influence of comprehensive stress (difference between verticle stress and side constraint) and verticle strain on the deformation modulus of pile in composite foundation . It is pointed out that the side constraint imposed by soil around the pile depends on the soil type and its state of stress. Based on the interaction between pile and soil, analyses are made on three different kinds of loading test on composite foundation composed of pile group, composite foundation composed of single pile and single pile respectively. The computation formula for the deformation modulus of pile is established accordingly.