To investigate the internal force and deformation response law and reinforcement effect of a shield tunnel reinforced by channel steel in a pit excavation, a self-developed “hydraulic loading system” is used to conduct a full-size test and numerical simulation analysis on a three-ring staggered-seam assembled tube sheet. The results show that the internal force response of the tube sheet on the excavation side of the pit is complex and sensitive, the internal force is transformed in the direction under different load levels, and the bearing capacity can be effectively increased by 62.5 % through the channel steel reinforcement; the channel steel fails due to weld cracking, and the nonstructural bonding surface is damaged by sliding; and the maximum reinforcing efficiency of the combination of the channel steel and steel plate reaches 45.8 %, making it an economical and reliable reinforcing program.