How to verify certification in FPTS?

Project:lftp (project)
Category:support request
Project wiki:lftp

I have a FileZilla FTP sever supporting FPTS. And I am trying to use lftp to login the server over FTPS mode with certificate. As the document said, I configured the lftp as bellow:

set ssl:check-hostname yes
set ssl:verify-certificate yes
set ssl:ca-file "path to your a certificate bundle file, containing the certificate authorities you choose to trust"

But I am confused about the ca-file. Where can I get the ca-file? I used the certificate file generated by the server. But it caused the information in debug log:
---- Resolving host address...
---- 2 addresses found: ::1,
---- Connecting to localhost (::1) port 990
Certificate depth: 0; subject: /CN=localhost/C=44; issuer: /CN=localhost/C=44
ERROR: Certificate verification: self signed certificate
**** SSL_connect: self signed certificate
---- Closing control socket

Can I accept the certificate generated by server as I did in WinSCP?