This paper presents a stable node-based smoothed finite element method (SNS-FEM) based on the upper theorem for stability analysis of a square tunnel in cohesive-frictional soils subjected to seismic forces and surcharge loading. The soil behavior is considered homogeneous Mohr-Coulomb with cohesion c, friction angle ϕ and unit weight γ. Using the pseudo-static approach, the horizontal kh and vertical earthquake acceleration coefficients kv are applied to the entire soil mass. Then, the square tunnel is subjected to horizontal and vertical surcharge loadings on the ground surface. The variations of the seismic stability numbers with the change coefficients kh have been investigated for different values of the cover depth-to-width ratio of the tunnel H/B, soil properties γB/c, and internal friction angle ϕ. Several numerical results of square tunnels have been investigated, showing that the SNS-FEM approach can demonstrate accuracy and efficient solutions. The seismic stability numbers results are presented in design tables and charts for engineers to use in the preliminary design stage of the square tunnel. Finally, the article investigates the effect of horizontal and vertical seismic acceleration components on the corrective coefficients of a square tunnel.