To investigate tunnel face stability under complex working conditions, a three-dimensional active failure model incorporating soil non-homogeneity and seismic effects is constructed based on the linear Mohr-Coulomb criterion, utilizing spatial discretization and "point-generation" techniques. A non-homogeneity coefficient is introduced to characterize the linear distribution of cohesion with depth, and an analytical upper-bound solution for the limit support pressure is derived and optimized via MATLAB. Comparative validation demonstrates that the proposed method possesses good calculation accuracy. The results indicate that seismic effects significantly reduce tunnel face stability; the limit support pressure increases with the rise of the horizontal seismic acceleration coefficient and the vertical seismic proportionality coefficient, with the vertical seismic action exhibiting a non-linear amplification effect. Conversely, soil non-homogeneity contributes to enhancing self-stability capabilities; the limit support pressure shows a significant linear negative correlation with the non-homogeneity coefficient, suggesting that fully utilizing the strength increase characteristics of deep soil can optimize support design. However, strong seismic action weakens the safety benefits derived from soil non-homogeneity, necessitating increased seismic redundancy in engineering designs for high-intensity earthquake zones.