Taking a case of Changsha Metro Line 4 excavated beneath closely spaced existing Changsha Metro Line 2 twin tunnels as research background, this paper studies the application of the Metro Jet System (MJS) Horizontal Column Reinforcement in Shield Tunneling. The quality of MJS column and the response of stratum and overlying tunnels are studied during the shield tunneling by carrying out in-situ and laboratory tests, installing the additional stress sensors and vertical displacement monitoring points inside Metro Line 2 and setting up monitoring points on ground surface. The result indicates that the diameter is appropriately 2 m and the compressive strength is higher than 3.0 MPa, which satisfy the designed specifications. When the shield excavates beneath the overlying tunnels, the settlement of the overlying tunnel present Gaussian curve distribution, and the maximum settlement is 4.33 mm. Due to the unloading of the shield excavation，the maximum additional stress is 1.4 MPa and the maximum settlement is 1.1 mm. Surface settlement is relatively small in the MJS horizontal pile reinforcement area. Overall，MJS horizontal pile can effectively reduce the settlement of existing tunnel，and can offer reference for the application of similar engineering.