Sea cucumber (Apostichopus japonicus) is one of the most important economic species of mariculture in north China, but the frequent outbreaks of various diseases such as skin ulceration syndrome hindered the sustainable development of A. japonicus aquaculture. As an important part of disease prevention and control, probiotics will hopefully replace antibiotics and be widely used in aquaculture industry. This experiment was conducted to investigate the effects of Lactobacillus plantarum LP HMX-3 on growth performance, digestive enzyme activity, immune ability and intestinal flora of A. japonicus. A total of 270 A. japonicus with an average initial body weight about (4.22 ±0.05) g were randomly divided into 3 groups, three replicates for each group, 30 A. japonicus for each repetition. The basal diet was used as control group (C group). Two experimental diets were obtained by adding 10⁵ CFU/g (LL group) and 10⁷ CFU/g (LM group) LP HMX-3 to the basal diet. The feeding experiment lasted for 8 weeks. The experimental results show that ①The final body weight, weight gain rate and specific growth rate of juvenile sea cucumber A. japonicus were significantly increased by adding LP HMX-3, and the LL group had better growth index. ②Compared with control group, the intestinal amylase activity in LL group and the intestinal lipase activity in LM group were significantly enhanced, while there was no significant difference in intestinal trypsin activity among all groups, but the value of probiotics groups was higher, indicating that LP HMX-3 may improve the digestion and absorption of nutrients in A. japonicus. ③Compared with control group, the activities of alkaline phosphatase and catalase were significantly increased in the group of LL and LM, and the acid phosphatase activity was significantly increased in the group of LM. There was no significant difference in superoxide dismutase activity among all groups. Besides, the mRNA expression levels of Aj-p105 and Aj-catalase in LL and LM groups were significantly increased compared with that in control group, and the mRNA expression level of Aj-C3 in LM group was also significantly higher than that in control group. These results demonstrated that dietary with L. plantarum LP HMX-3 supplemented could enhance the immune capacity of A. japonicus. ④All alpha indices suggested that LP HMX-3 significantly increased intestinal microbial richness and evenness, but the level of addition had little effect. The Beta diversity analysis indicated that the bacteria communities in probiotic supplemented groups were quite similar, while different from control group. ⑤At the level of phylum, the relative abundance of Proteobacteria in control group was significantly higher than any other groups, while that of Firmicutes in LL group increased significantly compared with control group. The relative abundance of Bacteroidota, Cyanobacteria and Campilobacterota was significantly increased in the group of LL and LM, compared with those in control group. At the genus level, the relative abundance of Lactobacillus, Bifidobacterium, Streptococcus and Clostridium in LL and LM groups was higher than that in C group, indicating that LP HMX-3 significantly increased the abundance of potential probiotics. The results of the study suggested that adding L. plantarum LP HMX-3 to the diet could effectively promote the growth of A. japonicus, improve its digestion and immunity capacity to a certain extent, and actively regulate the intestinal microflora of A. japonicus. In summary, the results of this study confirmed the probiotic effect of L. plantarum LP HMX-3 in A. japonicus, and provided technical support and scientific evidence for its application in aquaculture.