To investigate the stress characteristics of the primary support in the shallow buried biased section of the tunnel in fully weathered granite strata. Based on the Xiangsishan Tunnel, laboratory tests were conducted to determine the mechanical parameters including cohesion and internal friction angle of fully weathered granite. The construction process was simulated by FLAC 3D numerical software for the shallow buried bias section of the tunnel entrance. To study the effects of varying ground slopes, tunnel depths, and soil-rock interface locations on the stress characteristics of the primary support structure. The results indicate that, under various impact factors, the axial force and bending moment of the primary support exhibit the distribution characteristics of “upper large and lower small”, while the safety factor displays the opposite trend. The internal stresses of the primary support are distributed asymmetrically as the ground slope increases. The positive bending moment shifts toward the deeper-buried side and gradually increases, while the negative bending moment and axial force increase at the arch waist on the shallow-buried side. When the ground slope reaches 40°, the safety factor of the primary support falls below the standard allowable value. However, the stability of the primary support can be enhanced by implementing multi-stage variable slopes. As the depth of the tunnel increases, the asymmetric distribution of internal stresses in the primary support decreases, while the overall magnitude of internal stresses continuously increases. When the soil-rock interface crosses the tunnel at various locations, the magnitude of the internal stresses in the primary support is significantly affected, whereas the distribution pattern remains relatively stable. The reliability of the stress characteristics of the primary support under various impact factors was verified through on-site monitoring of the stress variations in the steel arch and concrete at various locations of the tunnel.