<p>Replacing rolling bearings with journal bearings can enhance torque density in wind turbine gearboxes. However, time-varying aerodynamic loads transmitted through the drivetrain induce complex coupled structural deformations, potentially leading to edge loading in planetary gear journal bearings (PGJB) and compromising system reliability. This study develops a multi-flexible-body dynamic model for a 6 MW wind turbine drivetrain, incorporating elastic support of the main shaft system, component-level structural flexibility of complex geometries, and PGJB interfacial compliance, validated against experimental. To reduce computational burden in load spectrum analysis, a novel high-efficiency envelope sampling (ES) method is proposed for load spectrum construction, significantly reducing the number of load cases while preserving critical load characteristics. A systematic investigation is conducted on the transient lubrication performance of the PGJB under varying operating conditions. Results show that, compared to the conventional load duration distribution-based spectrum, the ES-derived spectrum better retains the probabilistic distribution features of the raw load dataset. Furthermore, input torque strongly influences lubrication: as torque increases from 20 to 80% of rated capacity, the likelihood of edge contact rises markedly; beyond 80%, persistent edge contact occurs throughout the operational cycle, indicating a high risk of localized contact and potential lubrication deterioration.</p>