OK,first I have to apologize for my terrible English, 
,some of my words are translated by google！
I have asked the same question in a Chinese forum but no one knows.
Now I use a huawei router its very old.There are 50 to 60 users in our company,sometimes its reboots itselsf and it will make me crazy.Not long ago I got a intel P4 computer and installed ROS3.3 thought it is old either but I think it is enough.
About the huawei router I use a NAT POOL.Here’s 8 IPs from 114.xxx.xxx.16/29 to 114.xxx.xxx.23/29,as we know 16 and 23 can not be used.Here’s another 2 IPs 61.xxx.xxx.17/30 and 61.xxx.xxx.18/30.At first it make me confused but now I know that 61.xxx.xxx.18 can be used as a address and 17 as a gateway.Configured one computer with this two IPs(17 and 18) I can connect to the internet.If use a router I configure this two IPs to the network card and with a NAT POOL that IPs 114.xxx.xxx.17 to 114.xxx.xxx.22 in it,I can connect the internet either!So at the beginning I thought if I did the same things to the ROS3.3 it shoud be work.Then I configured the network card with IP address 61.xxx.xxx.18/30 gateway 61.xxx.xx.17 and a src-nat with IPs 114.xxx.xxx.17-114.xxx.xxx.22/29 and for the lan I used 192.168.1.0/24 but it did not work! 
And I dont konw why. 
For sure there is no problems with my hardware and for the src-nat I did a test in my local network.In the test I used IP address 192.168.1.92/24 gateway 192.168.1.1 for the network card(wan) and a src-nat with IP 192.168.1.92,about the lan I used 192.168.2.0/24.I know its not the same as the real condition but works fine!
I tried src-nat dst-nt masq none of this can be worked.
God!If anyone help me!Thanks a lot!!!
do you have a default route?