Sandy braided rivers can create extensive oil and gas reservoirs, with channel bars representing predominant sedimentary features. These bars, including compound middle channel bars and compound sidebars, exhibit complex internal architectural patterns resulting from multiple episodes of erosion, cutting, and redeposition during formation. To address these complexities, numerical simulations of sedimentation were employed to replicate the growth and evolution of the bars, enabling the analysis of repetitive sedimentation and erosion-cutting processes shaping their architecture. The results indicate the following: \mathrm{①} compound middle bars experienced downward migration followed by lateral migration due to water flow from both sides, whereas sidebars underwent lateral migration first and then downward migration due to water flow from one side; \mathrm{②} compound middle bars developed through downstream, lateral, and vertical accretions, whereas sidebars formed through lateral, vertical, or infilling deposits, all from bottom to top; and \mathrm{③} compound sidebars exhibited greater variation in the scale of lateral accretions compared to middle bars, displaying multiple generations and intricate interleaving relationships. A deeper understanding of the internal architecture of middle and sidebars provides novel insights into the characterization of underground oil and gas reservoirs.