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
can we conclude 
timeout server means the response time for haproxy server and the request server(app).
timeout client means the response time for specific backend server ?
if there are multiple backend servers for haproxy, backend server (group) A are dealing with the requests are very quickly, we can set the timeout client with a low value. for the backend server (group) B , which will respond lowly bacause sometimes cpu usage will be very high duing gabbage collection time, then we can configure it with a high value ?
timeout client is for the client side (between client and haproxy). That’s why you can set it in the frontend but not in the backend section.
timeout server is for the server side (between a backend server and haproxy). That’s why you can set it in the backend but not in the frontend section.
You can set different “timeout server” to specific values for each backend.