Greetings to you, zijin_cheng!
It seems that you've put some effort into your post in order to get some real help here. Would you please tell me which wifi hardware you use?
And with all respect to Backtrack... do you JUST use the LiveCD or do you have other Operating Systems installed as well?
Tell us something about you, boy...
I have an Intel Wifi link 5100 and I have windows vista 64 bit installed in my whole 320gb hd (nfts i think) and i am booting backtrack 4 beta from a usb stick without persistent changes. the only os i have installed on hd is vista.