Why does mysql slave can't connect to master by itself but command in shell can

I'm adding a slave node to my replication, there's a user in master named repl. After the configuration, my slave can't connect to my master. I paste some important log from my slave status:

mysql> show slave status\G
lave_IO_State: Connecting to master
Master_User: repl
Master_Port: 6693
Slave_IO_Running: Connecting
Slave_SQL_Running: Yes
Last_IO_Errno: 2003
Last_IO_Error: error connecting to master 'repl@172.31.6.63:6693' - retry-time: 60  retries: 19
Last_IO_Error_Timestamp: 150204 17:33:01

The slave is always trying to connect to master, but in vain.

However, I can connect to master in shell with mysql -u repl -h HOST -P 6693 -p . I don't know what's the problem. And it seems there's no log or other information about the connection error.

Any idea about this problem?