In order to determine the ecological function of seagrass Zostera marina L. in sea cucumber culture, water temperature and dissolved oxygen in the eelgrass bed of a sea cucumber pond were determined and compared with the adjacent sandflat. Food sources used by sea cucumber Apostichopus japonicas were determined by comparing their stable isotope signatures(δ¹³C, δ¹⁵N) with those of food source. Proportions of different carbon sources were estimated using the SIAR mixing model on δ¹³C and δ¹⁵N values. The results showed that water temperature at eelgrass patch bottom was 0 to 0.33℃ lower than the naked sandflat. Surface and bottom dissolved oxygen showed no differences between low eelgrass coverage(0 to 25%) area and sandflat, while in area with mid-to-high eelgrass coverage, resolved oxygen was 0.12 mg/L~0.62 mg/L higher in surface water. Bottom dissolved oxygen was lower in mid-to-high eelgrass coverage(0 to 0.38 mg/L). Stable isotope signatures δ¹³C and δ¹⁵N of sea cucumbers A. japonicus was -15.27% and 8.11‰, which differed from those in adjacent coastal seagrass habitat(-20.12% and 10.95‰). A strong role of Z. marina matter as food sources was indicated from the estimation of SIAR mixing model. The model gave higher upper and lower limits of 0.95 credibility interval(CI)(from 13% to 52%) for Z. marina relative to microphytobenthos, particle organic matter POM and epiphyte, for which 0.95 CI ranged from 4% to 49%, 23% to 39% and 0 to 22%, respectively. However in adjacent coastal seagrass bed A. japonicus showed different stable isotope signatures with -20.12‰ and 10.95‰ for δ¹³C and δ¹⁵N, respectively. 0.95 CI of SPOM and brown seaweed ranged from 24% to 54% and 1% to 53% while seagrass Z. caespitosa matter only contributed to 0~27%. These results demonstrate that Z. marina could reduce the bottom temperature in the pond and increase surface dissolved oxygen. Cultured A. japonicus consume relatively higher amounts of Z. marina than other organic sources in this pond. This implied the importance of seagrass habitat in the pond environment. The importance of seagrass habitats in sea cucumber pond was discussed and the necessity and feasibility to transplant seagrass to sea cucumber pond was highlighted.