Introduction: Improving pregnancy outcomes for women with diminished ovarian reserve (DOR) is a key area of focus in assisted reproductive technology (ART). Objective: This study aims to investigate the association between 25-hydroxyvitamin D (25-[OH]D) levels in follicular fluid (FF) of women with diminished ovarian reserve (DOR) and embryo quality during in vitro fertilization (IVF), and to perform a metabolomics analysis. Methods: A total of 54 women with DOR and 62 women with normal ovarian reserve (control) were enrolled. Based on FF 25-(OH)D levels, both DOR and control cases were stratified into two subgroups: the vitamin D non-deficient (VDH) and vitamin D deficient (VDL). Baseline characteristics and IVF laboratory outcomes were compared among groups and subgroups. Untargeted metabolomic profiling of FF was then performed to identify differences between subgroups. Results: FF vitamin D levels were significantly lower in the DOR group than in controls. In both cohorts, the normal fertilization rate was significantly higher in the VDH subgroup than in the VDL subgroup. Metabolites significantly upregulated in the VDH subgroup included phospholipids, hydroxychloroquine, and serinyltryptophan. Kyoto Encyclopedia of Genes and Genomes pathway enrichment analysis revealed significant enrichment in glycerophospholipid metabolism in the VDH subgroup. In the DOR group, the DOR-VDH subgroup exhibited upregulation of steroid hormone biosynthesis and vitamin metabolism pathways, particularly those mediated by cytochrome P450, relative to the DOR-VDL subgroup. Conclusion: Vitamin D may enhance the follicular microenvironment in DOR patients primarily by regulating lipid metabolism, thereby improving oocyte and embryo quality and developmental potential.