I can't pair my Apple Watch
my apple id is hacked and my trusted phone number is changed on his hacker. now my apple watch is cannot pairing
Change your Apple ID password. The only way anyone can "Hack" your Apple account is if you gave away your password in a phishing attack.
If you cannot sign or change it then you really are in trouble - contact Apple Support directly.
You should be able to reach them here
Click here > https://support.apple.com/contact
or phone numbers
Click here > https://support.apple.com/en-us/HT201232
Warning if you are just trying to hack into a watch you found or bought from a guy in a bar then don't even attempt to contact Apple Support.