Dear Yajun,
The required time step is intrinsically related to the structural natural frequencies that you are modeling. By disabling all DOFs except for the platform, the highest natural frequency is quite low, allowing for a large time step. Enabling DOFs introduces higher natural frequencies into the model, requiring smaller time steps. My rule of thumb for selecting the time step for the structural module is given in the following forum topic: FAST Integrator, step size & precision - #3 by Jason.Jonkman.
Best regards,