The invention discloses a novel focal plane diaphragm. The novel focal plane diaphragm is small in size. The novel focal plane diaphragm includes a diaphragm body with a diaphragm hole formed at the center; the incident surface of the diaphragm body is provided with a special-shaped groove around the diaphragm hole; a side surface end of the diaphragm hole, which is surrounded by the special-shaped groove, is a reflecting mirror surface with a certain inclination; and the reflecting mirror surface extends out from the special-shaped groove by a certain distance, so that the path of the reflected light of the reflecting mirror surface can be located outside the special-shaped groove. The novel focal plane diaphragm of the invention can effectively meet the use needs of the telescope of a Cassegrain system, provide a field diaphragm for a terminal instrument, keep a monitoring light path of a reset view of field, and can achieve a thermal control function required by a solar telescope.