An airflow-guiding device for a light source includes a cap and a guiding element. The cap has an airflow inlet, a through opening, an airflow outlet, a foundation, an annular side wall, and a cover. The foundation is connected to a reflector of the light source. The through opening is located at the foundation and corresponding to a light outlet of the reflector. The annular side wall where the airflow inlet is located is disposed on the foundation. The cover where the airflow outlet is located is disposed on the annular side wall and opposite to the foundation. The guiding element includes a guiding opening and an annular body. The annular body is rotatably disposed in the cap. The annular body where the guiding opening is located surrounds at least part of the through opening and forms a guiding tunnel together with the annular side wall in the cap.