Turning existing civil air defense tunnels into Metro tunnels is one of the focus and difficulties in the field of underground works. During the construction of Harbin Metro, an existing civil air defense tunnel, which has already been turned into a 4floor commercial mall of caisson structure, has to be renovated into a Metro tunnel under the condition that the safety and normal operation of the commercial mall are guaranteed. In the paper, numerical simulation models are established on basis of the load distribution, structure stress and deformation of the underpinning section, and relevant calculations and analysis are made. Then, a series of safe and effective underpinning program and deformation monitoring program are prepared on basis of the stress and deformation of the underpinning section and the analysis results. The construction practice shows that the analysis made is accurate, the construction program prepared is reliable, and the safety of the underpinning construction is guaranteed.