With both the high efficiency and economic advantages, the incremental launching has been widely used in bridge construction. As the steel U girder of the composite bridge has not formed the designed section during the launching construction, it has small stiffness relatively and poor torsion resistance ability. The structure mechanical behaviors are complex. Bridge structure collapsed during the incremental launching construction had been reported. Real-time strain monitoring of the adverse sections of the structure during the launching has important significance. By taking the Hangzhou Jiubao Bridge as background, this article introduced the real-time strain monitoring of the steel U girder and analyzed the monitoring results. Effective mastery of the safe state of the structure during launching construction was achieved. It had ensured the launching construction finished successfully. The research results can provide a reference for the design and construction of other similar engineering.