Abstract: The root foundation,a new type of foundation with variable cross-section,was introduced.The mechanism of vertical bearing capacity root foundation was analyzed,and the improvement of bearing capacity due to the soil compaction and the bending resistance of roots was discussed.The self-balanced test method was utilized in the field tests to investigate the vertical bearing behavior of a practical root-foundation and an experimental one.The distributions of axial force,side resistance and bending moment of roots were presented.Compared with the traditional caissons,it was indicated that the vertical bearing capacity of root foundation could be increased by 85%¹15%,and the settlement was decreased by 40%⁷0%,and the ratio of side resistance could be improved by 35% 45% under the experimental condition.