A large section of the shallow buried tunnel passes through a busy urban road and a viaduct, and is 1.65 m from the pile foundation, making the project extremely dangerous. The three-dimensional numerical calculations are therefore used to study the settlement pattern of the existing structures and the tunnel construction process, and to optimise the tunnel construction parameters to ensure the safety of the project. The results reveal that: (1) When the tunnel is excavated using the double sidewall guide pit method compared to CRD, settlement of the tunnel, road surface and bridge abutments is significantly reduced, and the displacement value is further reduced when the guide pit is excavated in 3 steps compared to 2 steps; (2) Changing the excavation sequence of each tunnel section has little effect on the settlement of the tunnel and road surface, and only the left and right guide pits are excavated alternately, compared to the left and right guide pits excavated sequentially under the abutment, settlement increases significantly; (3) Reducing the tunnel excavation footage can effectively decrease the degree of deformation of the tunnel and existing structures, but the effect is weakened after the footage is less than 0.8 m; (4) If the tunnel adopts the double sidewall guide pit method (3 steps), the excavation footprint is 0.5 m and the guide pits are excavated sequentially from left, right and centre, the settlement value of the tunnel, road surface and bridge can meet the control standard and there is a surplus. This study can provide a reliable reference for the safety control during the construction of large-section tunnels under various complex conditions in shallow buried areas.