Dear @Lei.Xue,
The thrust offset you are seeing is not the result of buoyancy (which is important for under-water turbines, but negligible for a wind turbine), but the result of the weight and inertia included in the ElastoDyn output RotThrust, which are not accounted for in AeroDyn’s output B1AeroFx, etc.. See my post dated Feb 27, 2020 in the following forum topic for my answer to a similar question for additional discussion on this thrust offset: NREL 5-MW reference turbine - CP, CQ, CT Coefficients.
Best regards,