To study the strain monitoring performance of weak grating array technology in the main beam of a cable-stayed bridge, the strain measurement of the steel truss main beam is carried out for the Yangtze River Bridge of Baijusi in Chongqing. Wavelength division multiplexing (WDM) and time division multiplexing (TDM) techniques were used for signal demodulation. Along the steel truss main beam, a strain measurement fiber with a grating spacing of 0.5 m and a temperature compensation fiber with a grating spacing of 2 m were laid in the measurement sections of the upper and lower chords. The strain state of the main beam during the bridge construction was then measured continuously over a 48-hour period. The results show that the weak grating array technology can collect data in real time in accordance with the predetermined acquisition frequency, and has good performance in the distributed strain monitoring of the main beam of the cable-stayed bridge. By combining these results with the temperature compensation scheme and monitoring design described in this paper, the influence of the construction state on the force of the main beam can be faithfully reflected. The results of the study provide a scientific reference for the use of weak grating array technology for the health monitoring of cable-stayed bridge structures.