The invention discloses a prefabricating method for a UHPC small box beam end diaphragm. A box beam body is prefabricated by adopting ultra-high-performance concrete, the diaphragm is prefabricated atthe end of the box beam body after the box beam body reaches the preset strength, a cast-in-situ material of the diaphragm adopts ultra-high-performance concrete or common concrete, retard-bonded prestressed tendon tensioning operation is conducted after the diaphragm reaches the preset strength, and finally high-pressure steam curing is conducted on the whole box beam body. The prefabricated boxbeam body is adopted and cooperates with post-poured UHPC or common concrete to form the diaphragm, the cast-in-situ end partition and the prefabricated UHPC box beam body can be effectively connected into a whole, the thickness of the diaphragm can be selectively adjusted according to the pouring concrete material, the prefabricating efficiency of the UHPC small box beam end diaphragm is effectively improved, and the prefabricating method is more suitable for bridge industry assembly line production.