Abstract: Peanutworm (Sipunculus nudus) obtains nutrients by ingesting surface sediments from tidal flats, digesting organic matter, and influencing the biogeochemical cycling of biogenic elements through various physiological activities that disturb the sediments. To investigate the sediment ingestion, energy metabolism, and energy allocation in S. nudus, we conducted a 191-day aquaculture experiment using four dietary treatments: mangrove leaf, green algae (Enteromorpha clathrata), mutual flower rice grass (Spartina alterniflora), and shrimp feed. We measured the individual ingestion rate (I), oxygen consumption rate (R_OCR), and ammonia excretion rate (E_AER) of S. nudus at different temperature stages, and then established an energy allocation model for S. nudus consuming the four diets. The results show that the diet affected the growth and survival rate of S. nudus significantly. The highest survival rate was obtained in E. clathrate group (90.67%), while the lowest was obtained in shrimp feed group (36.00%). E. clathrate group had the best growth performance, while mangrove leaf group showed the poorest growth. The oxygen consumption rate and ammonia excretion rate of S. nudus fed with different diets exhibited a quadratic relationship with temperature. The proportion of energy allocated to growth was relatively small, ranging from 6.57% (E. clathrate) to 14.36% (Mangrove leaf). The energy lost through feces ranged from 17.12% to 25.05%, while the energy lost through nitrogen excretion ranged from 19.70% (S. alterniflora) to 42.45% (E. clathrate). The proportion of energy consumed by routine metabolism was highest in S. alterniflora group (43.36%) but lowest in E. clathrate group (30.97%). Compared with other sediment-dwelling animals, S. nudus allocates a smaller proportion of energy to growth and a higher proportion to fecal and nitrogen excretion losses.