It works!I can use CakesFW on my N3DS now.
Thanks to @b1l1s,@Shadowtrance ,@mid-kid. I got my "firmware.bin" finally.
A little problem is that: in the emuNand system, I luanch the "Option" app, just to see the version number as "VerXXXXX", NOT "CakesXXXXX"(which surprise me:)and makes the cfw...