Dear @Danupon.Subanapong,
I don’t see a numerical instability in your heave response with SubDyn enabled, but there certainly is an undamped mode. In your model with SubDyn enabled, are you following the guidance I’ve outlined here: FeaCoupling of an External floater (platform+mooring) with Openfast #791 · Discussion #801 · OpenFAST/openfast · GitHub?
Best regards,