The load mode in the existing specifications does not fully reflect the interaction between backfill soil and structure. Mechanical analysis and theoretical derivation methods are employed to analyze the load mode of shed tunnel with soil backfill, and the load calculation formulas are derived to ensure the safety of the structural design. Numerical analysis and engineering cases are used to verify the results. Research results reveal the following: (1) The load of the backfill upper side of the structure, which transmits to the structure′s top plate in the form of additional horizontal and vertical forces with a triangular distribution, is ignored in the existing specifications. It can be calculated using the evolution formula of Coulomb earth pressure theory. (2) This additional load adversely impacts the structure, which worsens with increased backfill slope, especially on the shear force of the outer sidewall. (3) The case study results show that when considering additional load, the theoretical failure mechanism agrees with the actual failure mechanism. Thus, the influence of additional load must be considered in the structural design of the shed tunnel.