Abstract: In response to the 1st Aeronautics CFD Credibility Workshop, the CARDC CHN-T1 wing-body-tail configuration was analysed by two RANS solvers. A family of structured overset grids with equivalent parameters were generated. In these grids, four levels of grid size were used, and the number of grid cells from one level to the next refined level was increased by a factor of roughly 3.3. Based on these grids, studies were conducted regarding grid-convergence study and buffet prediction. Moreover, the results were compared with those from 1-1-blocking grid. The overset grid integrates the advantages of body-fitted grid and Cartesian grid. These advantages lead to radically reducing the generation complexity with compact blocking topology and superior near-field orthogonality. For the numerical simulation of transonic transport airplanes, overset grids have high resolution for shock waves and flow separations since more cells can be assembled on the wall compared with that of 1-1-blocking grids with the same mesh scale. The in-house code adopted in the present work possesses satisfactory computation efficiency and reliability, while the ability of NASA CFL3D solver in respect of overset grid needs further excavation and verification. The displayed results also include the difference between SA and SST turbulence model on predicted surface pressure distribution, separation bubbles and maximum lift coefficient. It is revealed that the model support in wind tunnel and the static aeroelastic deformation of the wing have great influence on the pitching moment of the model airplane.