The research on the sedimentation of the foundation in high-speed railway has been one of the concerned problems in civil engineering .Based on a railway foundation project in Tianjin; this paper puts forward a new method replacing the soft foundation with the lightweight foam soil to deal with the soft foundation. In this paper we also use ANSYS software to simulate and calculate the foun- dation which is replaced by the lightweight foam soil and get the sedimentation curves of the founds- lion under the different thickness of the lightweight foam soil. Through the comparison, we get the most economic and reasonable thickness which also meets the railway standards and it has practical guiding significance to this project.