Not too sure

持加拿大護照從中國入境美國，不須簽證吧？除了護照，還需什麽證件呢？
Correct if it's for visiting purpose(up to 6 months).

另外，綠卡持有者，若用加拿大護照免簽入境美國，是不是等於自放棄綠卡？
Not sure. Green card holders, regardless nationality, are supposed to identify yourself so upon arrival at port of entry. It's probably a gray area.