Mid-channel bars in sandy braided rivers, boasting a large scale, high connectivity, and favorable physical properties, serve as a significant type of hydrocarbon reservoirs. Complex and variable hydrodynamic conditions endow mid-channel bars with multiple types and complex internal architectures, which constrain efficient oil and gas exploitation. This study aims to explore the sedimentary evolution pattern and architectural characteristics of mid-channel bars in sandy braided rivers, with a specific focus on the influence of the sedimentary process. To this end, we conduct the dynamic simulation and process analysis of the sedimentary evolution of sandy braided rivers using a sedimentary numerical simulation method based on the real-time solution in hydrodynamic fields. The results are as follows: (1) Mid-channel bars in sandy braided rivers evolve in five stages, namely the sequential formation and continuous conversion of lozenge-shaped bars, tongue-shaped bars, unit bars, composite bars, and reworked composite bars. These bars differ significantly in planar morphology, cross-sectional structure, and scale; (2) Interactions between water currents and mid-channel bars act as the predominant mechanism governing the sedimentary evolution of sandy braided rivers. Specifically, the constant changes in the convergence and divergence characteristics and distribution styles of water currents facilitate the formation, accretion, migration, and deformation of the mid-channel bars, which are under frequent and complex superimposition and cutting. In turn, the evolutionary dynamics of the mid-channel bars further induces the above-mentioned changes in water currents; (3) Three types of accretion stemming from progradation, lateral accretion, and aggradation occur within the mid-channel bars. In the process from the formation of lozenge-shaped bars to the emergence of reworked composite bars, the accretion within mid-channel bars evolves from an initial dominance of progradation to the coexistence of progradation and lateral accretion, culminating in a combination of all three accretion types. During the transitional phase, the length and width of the mid-channel bars experience a rapid increase, followed by a slow increase, and finally stabilize. As revealed by sedimentary records, reservoirs of the mid-channel bar microfacies terminating at different evolutionary stages differ significantly in planar distribution pattern, internal architectural characteristics, and scale.