Thanks the answer from Justin Onisoru.
I have tried the DAMPED and QR DAMPED method. But the result is so strange, I got no any complex frequency.
I paste the operations here...
--------------------------------
/PREP7
!*
ET,1,SHELL63
!*
!*
R,1,4e-3,4e-3,4e-3,4e-3, , ,
RMORE, , , ,
RMORE...