For the special requirements of Thirty Meter Telescope( TMT) from USA, the Rotator assembly of M3S for the TMT was designed，and a design method was put forward for a rotation bearing when its load was varying. By analysis of the motion and force-giving modes of the system and comparison with large photoelectric telescope,a three-row roller bearing scheme was chosen and the model of bearing was established. The zenith angle was defined as a variable, and the most serious working condition was confirmed then the telescope position was determined at this moment. The bearing deformation and stress were calculated by the numerical calculation and Finite Element Method(FEM) when the telescope was stopped at this position. Obtained results from the two methods are in good agreement, which demonstrates the accuracy of the model. Experiments indicate that the maximum axial deformation on the single roller will not exceed 0.015 mm in x, y, z directions and the angular deformation is less than 1.7×10-5 rad when the azimuth varies in 0°-65°. The result shows a enough margin for design requirements.