As a critical component of high-end equipment such as aero-engines, the lubrication performance of cylindrical roller bearings directly impacts the operational reliability of the equipment. To thoroughly reveal the variation patterns of fluid drag loss within the bearing cavity under different rotational speeds and clearance conditions, this study constructs a high-precision fluid dynamics model of cylindrical roller bearings based on fluid dynamics theory. By introducing a rotating coordinate system, the motion relationships between the bearing components are accurately described. On this foundation, a research method for internal fluid drag loss in rolling bearings based on Computational Fluid Dynamics (CFD) is proposed, systematically quantifying the distribution characteristics of fluid drag loss within the bearing cavity. By varying key parameters such as rotational speed, pocket clearance, and guide clearance, the influence of these factors on fluid drag loss is thoroughly investigated, providing a scientific basis for the optimization of lubrication design in cylindrical roller bearings. This research offers essential theoretical guidance and support for bearing lubrication design and the enhancement of bearing operational reliability.