Hyphantria cunea is a widespread pest that causes significant issues in forestry and agriculture, with a far-reaching global impact. However, ultramorphological studies of H. cunea, especially in terms of ultramorphological changes, are far from in-depth. Therefore, we investigated the ultrastructure of its eggs, larvae, and pupae using scanning electron microscopy, focusing on ultrastructural changes in larvae across different instars. The eggs have rough surfaces, and the micropylar region consists of rosette cells. The larval head chaetotaxy is symmetrically arranged, and there are three types of sensilla on the antennae and mouthparts. The abdominal and anal prolegs feature a uniordinal heteroideus midband of crochets. The pupal abdomen has ten segments; in females, the genital slit (Fgs) is located at the anterior edge of abdominal segment VIII, while in males, the genital slit (Mgs) is situated in the middle of abdominal segment IX. First-instar larvae exhibit only primary setae, and secondary setae emerge in the second instar. As the larvae mature, the head, thoracic legs, and abdominal and anal prolegs all become larger overall. The surface of the verrucae is initially smooth, with microtrichia appearing from the fourth instar onwards, and the setae on the verrucae gradually increase in size. These findings provide additional features for larval species identification and contribute to the morphological study of H. cunea.