We investigated potential mechanisms underlying the beneficial effects of long-chain inulin against eczematous dermatitis in aging mice. The effects of long-chain inulin at mass fractions of 5% and 20% on eczema induced by D-galactose combined with 2,4-Dinitrochlorobenzene (DNCB) in aging mice were analyzed after feeding the mice for three weeks. Pathological changes in skin and colon tissues, inflammation-related factor levels in serum, spleen, liver, thymus, and colon tissues, and fecal microbiota were evaluated. Low and high dose long-chain inulin intervention alleviated skin damage and ameliorated pathological changes in colon tissues. Epidermal thickness decreased by 32.07% and 22.63% in low and high dose long-chain inulin treated groups, respectively. Expression of Interleukin-1 Beta (IL-1β) and Interleukin-6 (IL-6) mRNA in colon and the IL-6 and Interleukin-10 (IL-10) protein levels in serum, spleen, liver, and thymus were significantly decreased. The results of 16S rRNA sequencing showed that relative abundances of Verrucomicrobia, Verrucomicrobiaceae, Akkermansia and Clostridium were significantly higher in the model group. Intervention with different doses of long-chain inulin significantly decreased the relative abundances of these bacterial taxa, restoring them toward levels comparable to those of the blank control group. In comparison with the model group, the relative abundances of Bacteroidetes, Bacteroidaceae, and Bacteroides were significantly elevated, and the relative abundances of Firmicutes, Lactobacillaceae, and Lactobacillus significantly decreased after high-dose long-chain inulin intervention. These results provide a theoretical basis for long-chain inulin dietary intervention for age-related eczematous dermatitis.