Abstract: Lactococcus lactis has the ability to promote the host growth and immunity. In this study, L. lactis strain XP-15 isolated from the intestinal tract of sea cucumber (Apostichopus japonicus) was used as a probiotic candidate to study its effects on the growth, immunity and intestinal flora composition of sea cucumber. The sea cucumbers were fed with either basal diet (control group, namely CK group) or L. lactis strain XP-15-supplemented feed (10⁸ CFU/g, experimental group, namely XP-15 group) for 8 weeks. The results showed that the relative abundance of Lactococcus increased gradually to 54.00% in 8 weeks in the intestinal tract of sea cucumber in XP-15 group, higher than that of CK group (7.33% in weeks), indicating the colonization capacity of the strain XP-15 in the intestine of sea cucumber. The immune enzymatic activity assays performed at the 6th week showed that the activities of nitric oxide synthase, acid phosphatase and superoxide dismutase in the intestine of sea cucumber in the XP-15 group were significantly higher than those in CK group. In the 6th week, 30 sea cucumbers in either XP-15 or CK group were simultaneously used for a challenge test with Vibrio tubiashii (at 10⁸ CFU/mL by injection). As a result, the survival rates of sea cucumber in XP-15 group and CK group against V. tubiashii were 73.33% and 33.33%, respectively. In the 8th week, the weight gain rate in XP-15 group with initial weight of (43.63±2.15) g was 122.19%, which was higher than 109.46% in the CK group with initial weight of (42.10±1.93) g, indicating that the strain XP-15 had a certain promotion effect on the growth of sea cucumber. This study provides a reference for the application of Lactococcus lactis as a probiotic candidate in aquaculture.