Pre-support systems are very important for inner-city shallow tunnels in order to minimize twin tunnel-induced surface deformations. However, previous studies concerning the assessment of the magnitude of surface displacements caused by twin tunneling have not considered the effects of a pre-support system and stress release by deformation. The aim of this study is to introduce a procedure for obtaining modification factor including the effects of a pre-support system and the rock mass quality that can be used as a reduction ratio in the prediction methods used for twin tunnel-induced surface settlement. The data comes from Asian side of the Eurasia Tunnel excavated by the New Austrian Tunneling Method and supported by the forepoling and umbrella arch method. In this study, numerical analyses were performed on 12 cross-section lines along the tunnel route to update the geological profile, in which a parametric study with variable distance between pipes in the pre-support systems was conducted, and a statistical formula that presents the decreasing effect of pre-support system on maximum surface settlement was obtained. It is concluded that twin tunnel-induced surface settlement mainly depends on deformation modulus of the geo-materials around the tunnel. A new formula predicting twin tunnel-induced ground deformation is proposed as a modification factor of Herzog's equation.