Dear Yajun,
Increasing the time step to DT=0.25 s is likely leading to a numerical instability in the FAST solution. While you have disabled structural DOFs, one or more of the remaining 6 DOFs platform frequencies is likely high enough that a smaller time step is required. 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,