Dear Tianhui,
I’m not familiar with the PSD script you are using, but there appears to be a lot of windowing, which I do not believe results in a very realistic PSD in your case. Can you try a simpler PSD script, such as my script that calculates the PSD directly via FFT without windowing (but with optional binning to smooth the PSD). This script, Jason_PSD.m can be found in the following forum post: User defined spectrum from routine UserWaveSpctrm in HydroDYn - #7 by Jason.Jonkman.
Best regards,