Hi Nanzhe,
first, as always, you should swap to and install RAxML-ng, which is even faster than classic RAxML.
second, if you want to stick with the classic (stand-alone) version, you have to call not raxmlHPC executable but the ...-PTHREADS version, which is the parallising variant where you can set the number of cores to be used using -T
Cheers, G.