Step 1, Configuring BBB as follow shows:

route add default gw 192.168.7.1

vim /etc/resolv.conf  and add "nameserver 8.8.8.8" at the end.

Step 2, Configuring Ubuntu PC as following shows:

sudo su
#wlan0 is my internet facing interface, eth1 is the BeagleBone USB connection

iptables --table nat --append POSTROUTING --out-interface wlan0 -j MASQUERADE
iptables --append FORWARD --in-interface eth1 -j ACCEPT
echo 1 > /proc/sys/net/ipv4/ip_forward

Step 3,  Test the internet in BBB as bellow shows:

ping 8.8.8.8

ping www.baidu.com