Can we set a specific timeout time for a backend?

as we know , timeout is a global parameter , is there any to set to set a specific timeout time for a backend ?

No, timeouts are not a global parameters.

Many people put it into the default section, so they don’t have to configure it multiple times, but you can always specify the timeout in the specific section (client related timeouts in the frontend sections and server related timeouts in the backend section).

All of this is very clearly documented:

timeout server:
http://cbonte.github.io/haproxy-dconv/2.2/configuration.html#4-timeout%20server

timeout client:
http://cbonte.github.io/haproxy-dconv/2.2/configuration.html#4-timeout%20client

etc