The mined tunnel of Xiamen railway station of rail transit line 3 is 34.5 m in width and 11.7 m in depth. The tunnel crosses underneath Wucun commercial underground street, and the double lines laterally cross the BRT piers. Accordingly, the existing pile foundations of the stress and piers should be underpinned. As a result, countermeasures are put forward as follows: (1) The longitudinal wallbeam underpinning scheme is adopted to transfer the load of the underground street to the completed station structure and wall beams. (2) The reinforcements are embedded in special joints between the underpinning beams and the existing piles, and the tongues and grooves are cut. (3) The tunnel is supported by anchor bolts, and the weak soil layer on the sidewalls is grouted and reinforced. (4) The existing antifloating anchor bolts are removed and pipe roof is arranged within the tunnel entrance and exit. The measures adopted have effectively reduced the impact of tunnel construction on underground street and the safety of the tunnel, and the existing structures have been ensured. Finally, the numerical simulation results show that the longitudinal wallbeam system is reasonable, feasible, and safe.