A fluid-driven model, which couples the semi-implicit moving particle simulation (MPS) method with the finite volume method (FVM), was developed to investigate oil accumulation and gear churning power loss in a turboprop engine reduction gearbox under roll attitudes. Simulation results revealed that oil migration and distribution inside the gearbox cavity are predominantly governed by gear rotation effects. Under roll conditions, a circumferential shear flow field developed around the gears, along with a dynamic oil accumulation zone near the left deflector and a static oil accumulation zone adjacent to the curved deflector. Both the accumulated oil volume and gear churning power loss demonstrate a non-linear increase with increasing roll angle. An increase in gear rotational speed is found to suppress oil accumulation, yet it markedly amplifies churning power loss. Higher roll angular rate enhances the kinetic energy of the lubricant, thereby raising the average churning power loss during gearbox roll maneuvers. Validation conducted on a reduction gearbox test platform indicated that the maximum prediction error of the simulation model for the churning power loss in the split-torque gear stage is approximately 15%.