Characterizing ground deformation patterns for double-line shield tunnels beneath existing infrastructures proves to be both complex and technically demanding, which is easy to cause excessive ground deformation and poses serious threat to the normal operation existing tunnel. This paper relies on the new double-line shield tunnel of Xi’an Metro Line 5 undercrossing the existing operation tunnel Line 2. Firstly, the deformation characteristics of the strata during the construction of the new shield tunnel passing beneath the existing tunnel were investigated. Secondly, the study analyzed soil deformation behavior and existing tunnel mechanical performance across different undercrossing spacings. Finally, the optimal undercrossing spacing for double-line shield tunnels beneath existing tunnels was determined through this research. Analysis reveals the existing tunnel’s restraining effect on overlaying soil deformation decreases localized settlement but expands the affected disturbance area. Surface horizontal displacements display symmetric distribution relative to the tunnel axis centerline. After the construction of the rear tunnel, the symmetric point is slightly inclined to the side of the rear tunnel. A gradual stress increase is observed in the existing tunnel as undercrossing spacing diminishes. Undercrossing spacings below 1.2 m cause invert tensile stresses to overcome concrete strength thresholds, posing cracking hazards that may destabilize the existing tunnel. For safe construction, maintain 1.2–2.4 m undercrossing spacing between the new tunnels and existing structure, while implementing rigorous real-time stress monitoring of the existing tunnel.