Sipunculan worm (Phascolosoma arcuatum) is an emerging cultural species in recent years. Even though its artificial culture has been expanded, little is known about its biological characteristics. In this paper, the morphological structure and ultrastructure of Sipunculan worm were elucidated by hematoxylin-eosin staining method and scanning electron microscopy, respectively. The results showed that Sipunculan worm was mainly divided into introvert part and trunk parts. The trunk was armed with mastoid, while the introvert was flexible and could be elongated to the length about twice of that of the trunk. The introvert consisted of tentacles and nuchal organ (Fig. 1a). The digestive system of the Sipunculan worm was composed of four parts: mouth, esophagus, intestine and anus (Fig. 4), and the morphology of muscle tissues in the latter three parts was significantly different (Fig. 6). One terminal of the muscle was connected to the introvert (named introvert muscle), while the other terminal of the muscle was divided into 4 branches. Two of the branches were connected to the ventral part (named ventral retractor muscle), while the other two of the branches were connected to the dorsal part (named dorsal retractor muscle). The esophagus was attached to the introvert muscle and it was connected to the intestine. The intestine was divided into ascending loop and descending loop. The ascending loop and descending loop were winding and circling, the terminal of descending loop was linked to the rectum with a rectal blind sac, and eventually it was connected to the anus. There were two nephridium tubes existing in the ventral part (Fig.1b).