The eight old main brackets in a semi-submersible platform which connect columns with pontoons need be removed and replaced by new ones. It could be determined by Finite Element Analysis (FEA) that whether the operation of removing the old main brackets is safe or not, however, it is difficult to establish an accurate Finite Element Model (FEM) because of the uncertainty of external loading and supporting conditions for the semi-submersible platform in dry dock. A tailored cutting scheme is proposed to release the initial stress of an old main bracket, in which the main bracket and associated column and pontoon plates were partially cut. During the cutting procedure, the strains of 8 monitoring spots are measured by optic fiber grating sensors. The equivalent stress of 8 gauging points is calculated based on the Von Mises yield criterion, and the characteristics of stress variation in different locations is analyzed. The initial structural stress of all gauging points before cutting is obtained to improve the FEM with the calibrated loading and supporting conditions, which will be helpful to evaluate the feasibility of removing all old main brackets and provide the initial stress condition of the new main brackets for long-term stress monitoring.