Abstract: The bearing in Rotator assembly of TMT tertiary mirror system was designed, and the rule of parameter choice was discussed. Firstly, the load case of tertiary mirror system was analyzed, after that, the load-deformation equations of double-rows angular contact ball bearing were established and solved with Newton iteration method, the axial, radial and angular displacement were obtained, the load and deformation of every ball as also, the effect of different parameters were discussed at last. The result shows that, when the azimuth varies in 0-65, the displacement in optical axial direction is less than 0.022 mm, the displacement in rollaway plain is less than 0.042 mm, the maximum angular displacement is less than 3.5, the design requirement is satisfied, there are enough margin compared to the index. The load distribution equation and parameter choice rule of double-rows angular contact ball bearing can be applied in different cases.