To Dear all,
I have solved the problem!The problem is caused from the router of my school.I haven't mentioned that my pc and my board are in the same "WIFI" of my University, not my private "WIFI".
,Although I receive 0 package of data when I type ping www.google.com,the IP address of google just shows up. So it is accessible to the internet while we just can't ping to the IP out of my university.
One more thing is that, I can use wget to download something from the outside,so it seems does't matter.