The input function is a 60Hz square wave. From Fourier Transform I know that the frequency on the spectrum is 60,180,300....(2k-1). But there's a series of 120,240,360...(2k) signals appear. I've googled but all I found is Fourier Transform issues, not Fast Fourier Transform. I'm wondering why there are some 2k terms appearing on the spectrum. Thanks!
Looks like aliasing.
Your 360 Hz is probably not a real component at 360 Hz but it's the 9th harmonic (540 Hz) that aliases back to 360 Hz at a 900Hz sample rate.
That's easy to test: change the sample rate to, 940 Hz and see of the 360Hz moves to 400Hz or stays put. It moves to 400Hz, it's clearly aliasing.