Abstract: The combined steel support for foundation pits is a planar multi-legged lattice column， and there is a lack of systematic research on its in-plane stability of the imaginary axis. Various local standards and specifications for steel supports issued by different regions have inconsistent provisions for in-plane stability calculation. The formulas in current standards lack a rigorous theoretical basis for the shear effect on lattice columns and the lateral stiffness effect of steel columns， which may lead to unconservative calculation results. Based on a systematic research and analysis of the in-plane stability of lattice combined supports， this study proposed a critical load formula considering shear effects and boundary constraints. The cover plate of the steel support was treated as a deep beam， with its shear deformation effect taken into account. The actual connection between the support and the foundation pit was suggested to be modeled using fixed and simply-supported boundary constraints for calculation. The stabilizing effect of tie rods on the support was analyzed. Under the action of the lateral stiffness K of the steel column， it was found that the lattice column could exhibit multiple high-order instability modes with half-wave characteristics. The shear effect was considered for each mode. Accordingly， a critical load formula incorporating the effect of the steel column was derived. For long supports， two optimization measures and calculation methods for the combined arrangement of rigid columns were proposed.