Arabinoxylan (AX) has been found to improve an imbalanced gut microbiota. Lactobacillus gasseri is a beneficial endogenous bacterium that has been shown to have several health benefits in the human gut, particularly its lipid-lowering activity. However, it is not known whether AX can promote the action of L. gasseri. The results of in vitro experiments showed that AX promoted biofilm formation in L. gasseri, its acid and bile salt resistance could be enhanced, and enabled better colonization of L. gasseri in the intestinal tract of mice. In vivo experiments showed that the AX + L. gasseri group could effectively ameliorate weight gain and fat accumulation in high-fat diet-induced obese mice, and the L. gasseri group or AX + L. gasseri alleviated liver injury in mice. 16S rRNA sequencing showed that L. gasseri can colonize the mice intestine and AX + L. gasseri can ameliorate gut microbiota dysbiosis in obese mice by increasing Lactobacillus spp. and Coriobacteriaceae_UCG-002, and decreasing Peptococcaceae. In addition, metabolomics results indicated that the L. gasseri group and the AX + L. gasseri group could alleviate metabolic disorders by decreasing the levels of L-phenylalanine, L-tyrosine, kynurenine acid, and arachidonic acid in obese mice. The effect of AX + L. gasseri group was better than that of the L. gasseri group, suggesting that AX promotes the lipid-lowering activity of L. gasseri, and the mechanism may be due to the activation of retrograde endocannabinoid signaling. AX can be used as a functional food ingredient to potentially alleviate obesity and metabolic syndrome.