Hupehsuchus nanchangensis is the most representative species of the Nanzhang- Yuanan Fauna and one of the oldest known marine reptiles. However, information regarding the key cranial features of this species remains scarce. To address this gap, a recently collected fossil specimen was carefully repaired and studied using comparative anatomical methods. This study revises the following cranial diagnoses of this species: ① a narrow bony space develops along the midline of the skull develops along the two branches of the upper jaws; ② the premaxillae contact each other at their anterior ends, while the nasals only contact at their posterior ends; ③ a groove is present on the premaxilla, aligned with a similar groove on the maxilla; ④ the skull roof is flat and lacks a sagittal crest; ⑤ the parietal bone is larger than the frontal, and the pineal foramen is located in the anteromedial part of the parietal; ⑥ the upper temporal fenestra is formed by parietal, postfrontal, postorbital, and squamosal bones, excluding supratemporal; ⑦ the jugal is a triradiate bone with a thin, elongated anterior process and a short ventral process; ⑧ the supraoccipital is large and nearly circular; and ⑨ the exoccipital has clearly visible nerve openings. Comparative analysis reveals that Hupehsuchus nanchangensis, Nanchangosaurus, and Eretmorhipis all possess bony spaces formed between the upper jaws, which may represent a homologous character of Hupehsuchia. In Hupehsuchus nanchangensis, the supratemporal does not participate in the formation of the upper temporal fenestra. In addition, this species retains morphological vestiges of the jugal' s ventral process, and its inferior temporal fenestra is not fully reduced, resembling the condition in the oldest diapsid reptiles but different from the advanced ichthyosauromorphs. It is speculated that during the evolution of ichthyosauromorphs, the supratemporal gradually shifted anteriorly and became incorporated into the upper temporal fenestra, while the inferior temporal fenestra gradually closed. The new material also shows that the external naris of Hupehsuchus nanchangensis faces dorsally, and its bite force was relatively weak. These features suggest that this species had a unique feeding mechanism distinct from other ichthyosauromorphs.