In response to the limitations and one-sidedness of the simulation results of a rigid three-row cylindrical roller bearing for an offshore wind turbine main shaft under constant-load conditions, this paper proposes a simulation analysis method under variable loads. A contact mechanics model and a flexible body model are established, and the rigid-flexible coupled treatment is applied to the bearing’s inner and outer ring and cages. Based on variable load conditions, the theoretical speeds, simulated speeds, and acceleration responses of the pure rigid model and the rigid-flexible coupled model are compared, and the model is validated. Finally, the dynamic and transient responses reveal the time-varying characteristics of bearing loads and stress distribution patterns under different driving speeds and contact friction coefficients in the rigid-flexible coupled model. The conclusions are as follows: the rotational error of the rigid model is 1.67 to 3.76 times greater than that of the rigid-flexible coupled model, and the acceleration trend of the rigid-flexible coupled model is more stable with smaller speed fluctuations. The average forces on the thrust roller and cages increase with the driving speed, while those on the radial roller, cages, and inner ring decrease with the driving speed. The average force on the near-blade end cage is approximately 1.19 to 1.59 times that of the far end. The average force on the roller and cages significantly decreases with decreasing friction coefficient, with a reduction ranging from 50.08% to 76.41%. The maximum stress of the bearing increases with increasing driving speed. The novel simulation method for a rigid-flexible, coupled, three-row cylindrical roller bearing model under variable load conditions proposed in this paper can more accurately simulate the dynamic response of offshore wind turbine main shaft bearings during service. The results obtained in this paper provide highly valuable guidance for the research and design of offshore wind turbine main shaft bearings.