I found a solution. I get the user id with the WC_Order object and get the email user with it.
Here his my code :
$order = new WC_Order( $order_id ); // WC_Order object
$user_id = $order->get_user_id(); // get the user id
$user = get_user_by('id', $user_id ); // get the user by id field
$current_email = $user->user_email; // user mail