Its type should be char device as shown below.
-rw-r--r-- 1 root system 619851909 May 15 14:23 null 2>&1
crw-rw-rw- 1 root system 2, 2 May 15 14:24 null
after "rm null*" , it became
-rw-r--r-- 1 root system 41 May 15 14:24 null
-rw-r--r-- 1 root system 190 May 15 14:24 null 2>&1
something like a normal file.
Googled but those solution doesn't work, as I failed to delete /dev/null "file" although there is no error message returned.
e.g.
rm /dev/null ; mknod -m 0666 /dev/null c 2 2 ; chown root:system /dev/null
So my solution is to reboot the server, and it works for me since this is a test server.
#ls -l null*
crw-rw-rw- 1 root system 2, 2 May 15 14:56 null
-rw-r--r-- 1 root system 31458 May 15 14:56 null 2>&1
