Abstract: This study was to reveal the stress mechanism of load-bearing sliding supports in the complex dismantling conditions of old bridge structures during the renovation of large-span beam bridges, and to quantitatively analyze the impact of the dismantling process on the stiffness of the support system, in order to ensure construction safety and efficiency. Based on the actual form of temporary supports, four typical structural calculation models were constructed using finite element numerical simulation technology; By combining theoretical analysis, numerical simulation, and real-time on-site monitoring, a systematic study is conducted on the response laws of key mechanical parameters such as strength, stiffness, and stability of the support system. The research results indicate that during the process of bridge dismantling, the support system exhibits unique stress characteristics of "local strength, overall weakness, mid span stress concentration, and sensitive boundary constraints". The linkage between finite element simulation and real-time monitoring can effectively warn of stress concentration phenomena and significantly reduce structural risks. Numerical analysis confirms that optimizing the spacing between pier and column supports is a key measure to improve overall stability and achieve material economy. This study had clarified the mechanical behavior of this type of support under dynamic bridge dismantling loads. The proposed "simulation monitoring" dual control technology can achieve "controllability, visibility, and efficiency" in the construction process, providing important theoretical basis and a practical paradigm that can be promoted for similar bridge renovation projects.