Dear Chenxu.Zhao,
The FAST (ElastoDyn) output RotThrust is the reaction force transmitted from the rotor to the shaft, so, not only includes the applied aerodynamic forces, but also the forces resulting from rotor weight (and in transient analysis, rotor inertia); see related forum posts on this topic. So, normally, RotThrust is shifted upward relative to the aerodynamic applied Thrust. If you are using AeroDyn v15, you can output the aerodynamic applied rotor thrust via AeroDyn output RtAeroFxh.
Best regards,