I installed two system on one disk, archlinux + win7. I use dolphin to view win7's songs. But when I drag one to Amarok, it cannot play. If I start Amarok and select the song's location, it can play. I think maybe amarok can't recognize the /run/win7 location ?

Which exact Amarok and KDE versions is this about?

I changed the title of your questions as Amarok can play the tracks without problems when started from Amarok.

The dragging from outside is most likely a problem due to the underlying hardware layer. There are several open bugs about this in this list:
https://bugs.kde.org/buglist.cgi?quicks ... _id=570970

indicating a problem, but which layer this is caused by is hard to identify apparentlymostly solid.

Let me describe this situation in detail:
OS: ARCHLINUX
DESKTOP: KDEBASE 4.10
Harddisk: LINUX + windows7

Failed OP: I use dolphin to access win7's part, input root password. I drag one mp3 file in win7's part from dolphin to opened amarok. amarok can't play, without any info.

Successed OP: I use dolphin to access win7's part, input root password. I open file chooser from opened amarok, and select the same mp3 file in win7's part. (I noticed that win7 part was mount under /run ) This time, amarok can play the mp3 successfully.

So, is it a bug to amarok ? I remembered it won't happen in KDE3 times.

No, it is not a bug in Amarok, read my previous answer again. It's the drag action that is broken, that has nothing to do with Amarok, it is somewhere in the underlying system.