The fourth-level river terraces on the south bank of the Yellow River in Lanzhou are frequently affected by loess-debris flow landslides that threaten nearby communities. To clarify their kinematics and dynamics, a preloading DEM(discrete element method) was established in PFC-2D that incorporates gravitational stress and the Yunsheng Printing Plant landslide (north Fulongping) was used as a test case.Compared with the conventional “sand-rain” method, the preloading approach increased computational efficiency by up to 7.5× and produced a more realistic gravitational stress field, as evidenced by denser particle bonding and a more homogeneous force-chain network.The simulation faithfully reproduced the retrogressive failure process and the final deposit morphology observed in the field. Analysis of the micro-mechanical evolution shows that basin-scale disintegration of the particle skeleton and the consequent loss of inter-particle cohesion govern the catastrophic transition.The results validate the preloading DEM framework for loess-debris flow landslides and offer a reliable tool for future hazard assessment and mitigation design.