In order to meet the requirements of large-scale and real-time monitoring of the overall deflection of the main span, a box girder deflection measurement method based on ultra-weak fiber grating (UW-FBG) array is proposed. Theoretically analyzed the curve reconstruction principle based on grating wavelength-deflection, ANSYS simulation analyzed the error of strain inversion deflection, and the double-ended simply supported girder model with 4.8 m span was constructed by using UW-FBG arrays, and the maximum relative error between the experimentally tested inversion deflection and that measured by the micrometer was less than 6.80%. On the 340 m main span of Jinhai Bridge, 340 gratings with a spacing of 1 m were continuously deployed, and the real-time strain distribution of the bridge was monitored during the load test and the maximum was about 416.3 με. The real-time deflection of the bridge force was about -0.4010 m by using the curve reconstruction algorithm to invert the performance of the bridge force.