when i reinstall AutoStart,to rejoining it to the existing Autostart domain,
The right steps are delete the existing data source and recreate the Mirror data source ?the source is node B ?
Yes, first add the node to the domain, verify that the configuration is ok as far as the nodes go, and the take offline the resource group, delete the mirror data source from the resource group configuration, delete the data source, and create it again. Since the valid data at this moment is on node B you will select node B as the source node.
Then add the data source to the resource group (make sure the position in the startup and shutdown sequence is correct), and that bring online the resource group on node B. At this point the data source should start the initial sync, and until it is completed it will stay in warning state but you should see the percentage progress mark.
Once it syncs you can do a failback again (if the application or service is configured properly on the reinstalled node).