Lagrangian method is applied widely in geotechnical engineering in recent years. However, it doesn't deal with infinite domain problems well, which are frequently encountered in geotechnical engineering. Truncated boundaries must be used to slove such problems in Lagrangian method. Thus the Lagrangian method and infinite element method are investigated carefully first. Based on these, infinite elements are imported to the Lagrangian method and a coupling schema is proposed. In the schema, the center domain is treated with Lagrangian method and the boundary with infinite element. Explicit scheme is adopted for the calculating procedure of infinite element, same as the Lagrangian method, which spares the calculation of global stiffness matrix. Numerical convergence of the method is also disposed successfully. It is evidenced by some numerical examples that the proposed method can reduce errors on the truncated boundary obviously.