Is there any way to show a "computed field" value in edit mode?
So the solution found is :
I catched the node id from $form_state and then catched the field value from the node like this :
 /**
 * Implements hook_form_alter
 *
 */
function gedproject_form_alter(&amp;$form, &amp;$form_state, $form_id) {
  if ($form_id == 'node_structure_edit_form') {
    $form_object = $form_state-&gt;getFormObject();
    if ($form_object instanceof EntityForm) {
      $entity = $form_object-&gt;getEntity();
      $value = $entity-&gt;get('field_reference')-&gt;value;
      $form['field_reference']['widget'][0]['value']['#value'] = $value;
      $form['field_reference']['widget'][0]['value']['#type']='textfield';
      $form['field_reference']['widget'][0]['value']['#title']='Référence';
      $form['field_reference']['widget'][0]['value']['#attributes']['readonly'] = 'readonly';
    }
  }
}