<p>The increasing demand for paste backfill materials prepared from coal gangue drives the need for a deeper understanding of its close packing mechanism. In this study, paste backfill materials were prepared using coal gangue as aggregate, and the effects of three different close packing theories—Talbot theory, MAA (Modified Andreasen and Andersen) grading theory, and i method—on the mechanical properties and flowability of the backfill materials were systematically investigated. The phase composition, microstructure, and pore structure of the materials were subsequently analyzed to explore the mechanisms influencing their behavior in greater depth. The results showed that, according to the Talbot theory, the coal gangue backfill materials exhibited optimal overall performance when <i>n</i> = 0.44. Under the MAA grading theory, the optimal performance was achieved at <i>q</i> = 0.25. For the i method, the overall performance reached its peak when <i>i</i> = 0.74. Through fitting the torque-rotational speed curves, it was found that <i>i</i> = 0.74 better conformed to the Bingham fluid model. Furthermore, the pore structure and rheological properties were optimized by the appropriate incorporation of coal gangue powder. New insights and technical support were provided by this study for the large-scale utilization of coal gangue in the preparation of paste backfill materials.</p>