How to make one facet default? I want to set facet value default, when we search it will automatically checked 1st facet.
You should define hook_search_api_solr_query_alter and something like this:

/**
 * Implements hook_search_api_solr_query_alter().
 */
function MODULE_search_search_api_solr_query_alter(array &amp;$call_args, SearchApiQueryInterface $query) {
  $call_args['params']['fq'][] = 'FACET_MACHINE_NAME:YOUR_VALUE';
}


I hope it helped you.

The other way to do this described here http://www.eilyin.name/note/how-set-value-facet-default-drupal-54