This study made progress about Demage Mechanism of Transition Section in Cut and Cover Tunnel. For this study, Inspection and test was carried out about Transition Section. After this process, Numerical Analysis was accomplished by 2D, 3D. A result of inspection and test, It couldn't find the reason why the upper slab Demage was detected. So 2D Numerical Analysis was conducted. It was analyzed that the Safety Factor(1.0) was satisfied in 2D. But, the result of 3D Numerical Analysis, The reason was found that the Demage on upper slab was caused by moment change. The Moment was changed by column interval transition. For Retrofitting, Column was added under slab in tunnel. It was found that the addition column decreased upper slab deformation. After this study, It could be find that are important 3D Numerical Analysis as well as 2D Numerical Analysis in case of Transition Section. This Study can help developing construction and maintenance about Tunnel. Finally, It's going to study Retrofitting plans which have minimum influence of Transition Section in Cut and Cover Tunnel.