Steel and concrete composite structures are widely used in bridge engineering, for it can fully utilize the compression property of concrete and tensile behavior of the steel. However, the coupled behavior of shrinkage and creep exist in concrete. The creep behavior is dependent on the initial stress, while shrinkage is not. The shrinkage and creep of the concrete have a significant influence on the internal force and deformation and it may cause the cracking or even the failure of the structure. Nowadays, precast concrete slab is widely adopted in the composite bridges to reduce the effect of shrinkage and creep. Storage time is a critical parameter for the precast concrete slab to reach the best economic benefit and mechanical behavior of the structure. Therefore, in this paper, the finite element model of Xinshiji Bridge with the consideration of the relative slip between the steel and concrete was established to investigate the influence of loading age of the concrete on the mechanical behavior of the composite bridge, and the optimal storage time was determined.