ObjectiveTo elucidate the role of interleukin-33 (IL-33) in glucocorticoid-induced osteonecrosis of the femoral head (ONFH) in mice, with particular emphasis on its effects on bone remodeling, inflammatory regulation, and fibrosis. Methods In vivo: Fifteen 9-week-old male C57BL/6J wild-type mice were randomly divided into a normal control group, an ONFH group, and an intervention group, with 5 mice in each group. A glucocorticoid-induced ONFH model was established in the ONFH group and intervention group using a combined administration of lipopolysaccharide and methylprednisolone. The intervention group received intraperitoneal injection of IL-33 for 4 consecutive days during the early stage of model establishment; the normal control group received saline injection at the same time points. General conditions of mice were observed during the experiment. Endogenous IL-33 and transmembrane ST2 (ST2 ligand, ST2L) expression in the femoral head was assessed via immunofluorescence, quantitative PCR (qPCR), and Western blot. Bone necrosis and fibrosis were evaluated using HE and Masson staining. Immunohistochemistry was performed to detect osteogenic markers [osteocalcin (OCN), osteopontin (OPN), Runt-related transcription factor 2 (Runx2)] and osteoclastic marker (receptor activator of nuclear factor-κB ligand, RANKL), while serum cytokine levels [tumor necrosis factor (TNF-α), IL-6, IL-1β, IL-4, IL-10] were quantified by ELISA. In vitro: Murine osteoblasts were divided into control group (DMEM+PBS), IL-33 group (DMEM+10 ng/mL IL-33), and IL-33+ST2L group (DMEM+10 ng/mL IL-33+1 μg/mL ST2L antibody). After corresponding treatment, cell proliferation was detected by EdU incorporation assay. Additional osteoblasts were subjected to osteogenic induction culture, and mineralization, and the expression of osteogenesis-related genes (Runx2, collagen type Ⅰ, OCN, and OPN) were assessed by using alkaline phosphatase (ALP) staining, Alizarin red staining, and qPCR, respectively. Results In vivo: All animals survived until the completion of the experiment. Mice in the intervention group and ONFH group showed restricted mobility. Compared with the normal control group, the expressions of IL-33 and ST2L significantly upregulated at both mRNA and protein levels (P<0.05). Exogenous IL-33 administration exacerbated, rather than ameliorated, trabecular destruction and fibrosis, with the intervention group showing significantly increased fibrosis area percentage and empty lacunae rate compared with the other two groups (P<0.05). Furthermore, IL-33 treatment further suppressed the expressions of osteogenic markers (Runx2, OCN, OPN) while significantly enhancing the expression of the osteoclastic marker (RANKL) (P<0.05). ELISA results showed that compared with the ONFH group, serum levels of pro-inflammatory cytokines (IL-4, IL-6, IL-1β) were significantly lower in the intervention group (P<0.05). In vitro: Compared with control group, IL-33 significantly impaired osteoblast proliferation and differentiation, as evidenced by reduced cell proliferation rate, decreased ALP activity, and reduced calcium nodule formation (P<0.05). The expression of osteogenesis-related genes was also suppressed, with significant differences between groups (P<0.05). ST2L blockade effectively reversed these IL-33-mediated suppressive effects, leading to significant recovery of osteoblast proliferation and differentiation (P<0.05). Notably, the mRNA expression levels of collagen typeⅠand OCN were restored to normal (P>0.05). ConclusionIL-33 exacerbates ONFH by impairing osteoblast viability and function and inhibiting bone regeneration. Targeting the IL-33/ST2L signaling axis may represent a promising novel therapeutic strategy for ONFH.