I try to read the source.And i found the servers send XML like this：
<transport xmlns="urn:xmpp:jingle:transports:ice-udp:1" ufrag="7g0ug" pwd="50d2095drkciu1f1krjagldbqa"><candidate foundation="1" component="1" protocol="udp" priority="2130706431" generation="0" id="13" ip="fe80::c66a:b7ff:feae:7dc1" port="5002" type="host" network="0"/><candidate foundation="2" component="1" protocol="udp" priority="2130706431" generation="0" id="14" ip="192.168.1.117" port="5002" type="host" network="0"/><candidate foundation="3" component="1" protocol="udp" priority="2113937151" generation="0" id="15" ip="fe80::1043:4ff:fe14:523a" port="5002" type="host" network="0"/><candidate foundation="4" component="1" protocol="udp" priority="2113932031" generation="0" id="16" ip="10.0.2.15" port="5002" type="host" network="0"/><candidate foundation="6" component="1" protocol="udp" priority="1677724415" generation="0" id="17" ip="183.63.90.***" port="31430" type="srflx" rel-addr="192.168.1.117" rel-port="5002" network="0"/><candidate foundation="5" component="1" protocol="udp" priority="2815" generation="0" id="18" ip="121.40.147.***" port="30006" type="relay" network="0"/><candidate foundation="1" component="2" protocol="udp" priority="2130706430" generation="0" id="19" ip="fe80::c66a:b7ff:feae:7dc1" port="5003" type="host" network="0"/><candidate foundation="2" component="2" protocol="udp" priority="2130706430" generation="0" id="20" ip="192.168.1.117" port="5003" type="host" network="0"/><candidate foundation="3" component="2" protocol="udp" priority="2113937150" generation="0" id="21" ip="fe80::1043:4ff:fe14:523a" port="5003" type="host" network="0"/><candidate foundation="4" component="2" protocol="udp" priority="2113932030" generation="0" id="22" ip="10.0.2.15" port="5003" type="host" network="0"/><candidate foundation="6" component="2" protocol="udp" priority="1677724414" generation="0" id="23" ip="183.63.90.***" port="31560" type="srflx" rel-addr="192.168.1.117" rel-port="5003" network="0"/><candidate foundation="5" component="2" protocol="udp" priority="2814" generation="0" id="24" ip="121.40.147.***" port="30007" type="relay" network="0"/></transport>
So i think i can make Jitsi VideoBridge to force relay by change the response of servers,but i can't find where to change the code.
best regards.