Abstract: This study conducts a numerical investigation on fuel/air mixing and combustion characteristics in an inter-turbine combustor, with a focus on elucidating the influence mechanisms of fuel injection schemes on mixing dynamics and combustion control strategies. By introducing macro- and micro-mixing concepts, a numerical model is established using ANSYS Fluent software, where the FGM (Flamelet Generated Manifold) method is employed to simulate turbulent combustion processes. Transport equation analysis is implemented to characterize fuel residence time properties in the cavity recirculation zone. Four distinct fuel injection configurations (front-wall, top-wall, rear-wall, and combined injection schemes) are systematically designed and compared regarding their effects on mixing efficiency, combustion efficiency, and flame stability. Results demonstrate that injection schemes significantly alter fuel residence time in the cavity recirculation zone, with rear-wall injection (Scheme III) achieving the longest residence time (exceeding 4 ms), which enhances flame stabilization. The mixing process is predominantly governed by convection-dominated macro-mixing, while micro-mixing shows significant dependence on turbulent fluctuations. At a position 20 mm downstream of the afterburning zone outlet, the comprehensive mixing efficiency of Scheme 3 reached 98.5%, while that of other schemes at the same location was only 92%–94%. In terms of combustion performance, the overall combustion efficiency of Scheme 3 at the combustor outlet reached 95.1%, which is higher than that of Scheme 1 (93.2%), Scheme 2 (92.8%), and Scheme 4 (94.0%), further validating the advantage of its rear-wall injection in overall combustion performance. The combustion regime primarily follows diffusion flame characteristics, but strong turbulence induces local extinction and re-ignition phenomena, exhibiting partially premixed features. The cavity recirculation zone maintains flame stability through sustained forced ignition of combustible gases by high-temperature products, while the primary fuel oxidation and heat release processes occur in the mainstream channel outside the cavity. These findings provide theoretical foundations for optimizing fuel injection strategies and combustion organization design in inter-stage combustors.