<p>Rockbursts are critical geohazards during the construction of deep buried tunnels under high in-situ stress conditions. However, their accurate predictions and effective mitigations remain challenging in complex geo-stress environments. This study utilizes an in-house combined finite-discrete element method (FDEM) to conduct a case study on the rockbursts observed in a deep buried railway tunnel in southwestern China revealing their occurring processes and mechanisms under onsite high in-situ stress conditions. The model successfully reproduces the entire development process of typical rockbursts occurring on site, including fracture initiation and propagation, as well as fragment ejection and flyout, which is difficult to capture in field monitoring. The simulation results demonstrate that, due to the orientation of the in-situ stress, the rockburst initiates at the left spandrel of the tunnel and then progressively extends toward the tunnel crown and left sidewall, forming a V-shaped notch, which is consistent with field observations. Then, systematic parametric studies are conducted to further investigate the effects of lateral pressure coefficients and the deviation angle of in-situ principal stress orientations from the vertical direction on the rockburst behavior. It is found that the lateral pressure coefficient governs both the spatial location of the rockburst and the orientation of macroscopic fractures, while the deviation angle serves as a key factor controlling the migration of dominant rockburst damage zones along the tunnel profile. This study is expected to enhance the understanding of rockburst mechanisms in complex geo-stress environments and provide theoretical basis for risk assessment and support design of tunnels in deep underground.</p>