The analysis of the overall stress field distribution and state parameters of the main structure in building health monitoring is a bottleneck problem in the online state assessment technology. In order to comprehensively and consistently reflect the overall state information of the building, an optical fiber sensor network system was designed for the main structure of the building. A mapping algorithm was proposed, which established the relationship between the stress field information in the main structure and the position offset data of the overall structure. According to the theory of material mechanics and deflection, the deflection matrix was used for the conversion of stress field and displacement field. The simulation analyzed the distribution law of the stress field of the horizontal beam and the vertical beam in the main structure when the external load was applied, so as to provide a quantitative basis for the laying of the sensing unit. In the experiment, square steel and I-shaped steel were used to simulate the main structure. The load external stress range was 200-1000 N, fiber grating strain sensors were selected as sensing unit, and the same type of temperature sensors were used to compensate for temperature drift. The test results show that the intersection of the horizontal beam and the vertical beam is the most sensitive. The wavelength shift can well reflect the stress field distribution. The total amount of wavelength shift at this position is about 25 nm. At a gradient distance of 0.5 m, the total wavelength shift is 16 nm. The average error of the position offset of the main structure calculated is 7.46% and 6.34% based on the wavelength offset data. The average ratios of the corresponding stress to position offset for square steel and I-shaped steel are 1.99×10^-7 m and 2.79×10^-7 m, respectively. It can be seen that the system can test and calculate the overall health of the structure.