Finite element calculation of plane bar support system is an indispensable part of the design of foundation pit support, and the selection of boundary condition is especially important. To address the issue of unreasonable boundary setting in the finite element calculation of plane bar support system in foundation pit, a combined elastic boundary involving normal spring only subjected to compression and tangential spring is proposed in this article to investigate the influence of spring stiffness coefficient on the inner support force and deformation of foundation pit in finite element analysis. Results showed that by eliminating the influence of traditional constraints on the inner support force and deformation, the combined elastic boundary could result in a more accurate horizontal equivalent stiffness of inner support, and more precise internal forces. The inner support force has no concern with the stiffness coefficient of the normal spring only subjected to compression and the tangential spring. In addition, the displacement of inner support is controlled by the tangential spring rather than by the normal spring only subjected to compression. The proposed combined elastic boundary has clear physical meaning, and the value of spring stiffness coefficient is easy to obtain, hence could be applied to the design of foundation pit support.