Harbin Metro from West Bridge to Jiaohua Square and Jiaohua Square to the railway bureau is constructed by extending the existing civil air defense engineering. Aiming at its structural-broken security problems, the structure internal force variations for first breaking and after breaking are studied. By means of appropriately selecting the first-breaking point and breaking direction, interim vertical pre-support enforcing from sidewall to arch invert as well as vertical grouting and interval digging expanding, the structure internal force variations are within controllable range, and the vault settlement, arch invert floating and ground movement are effectively controlled. The structure and constructing security and the smooth project process are ensured.