〔Abstract〕 To investigate the mechanism through which miR-21 improves chronic renal fibrosis in mice via targeted modulation of the phosphatase and tensin homolog（PTEN）/protein kinase B（AKT）/mammalian target of rapamycin（mTOR）pathway. Methods Thirty-two chronic kidney disease model mice were randomly di- vided into four groups（n=8 each group）： model group，miR-21 overexpression group，miR-21 inhibition group， and miR-21 inhibition + MK-2206 group. Eight healthy mice were included as the control group. The miR-21 over- expression，miR-21 inhibition，and miR-21 inhibition + MK-2206 groups received tail-vein injections of lentivirus （50 μL，1×10⁸ TU per mouse）once weekly for three weeks. The control and model groups were injected with an equal volume of empty vector（LV-NC）. The miR-21 inhibition + MK-2206 group additionally received gavage of the AKT/mTOR pathway inhibitor MK-2206（480 mg/kg） once weekly for three weeks. The expressions of miR-21，24 h urinary protein，serum creatinine（Scr），blood urea nitrogen（BUN），and renal tissue levels of col- lagen I，collagen III，α-smooth muscle actin (α-SMA），and PTEN protein，as well as p-AKT/AKT and p-mTOR/ mTOR ratios，were compared among groups. HE staining was used to observe pathological changes in renal tissue， and Masson staining was used to observe the degree of renal fibrosis. A dual-luciferase assay was performed to verify the targeting relationship between miR-21 and PTEN. Results Compared with the model group，miR-21 ex- pression in renal tissue increased in the miR-21 overexpression group（P<0. 05）and decreased in the miR-21 inhi- bition group（P<0. 05）. Compared with the model group，the miR-21 overexpression group showed increased 24-h urinary protein，Scr，BUN，and renal tissue expression of collagen I，collagen III，and α-SMA（all P<0. 05）， while these indicators decreased in the miR-21 inhibition group（P<0. 05）. Compared with the miR-21 inhibition group，the miR-21 inhibition + MK-2206 group exhibited lower 24-h urinary protein，Scr，BUN，and renal tissue expression of collagen I，collagen III，and α-SMA（all P<0. 05）. Compared with the model group，the miR-21 overexpression group showed decreased PTEN protein expression（ P<0. 05） and increased p-AKT/AKT and p-mTOR/mTOR ratios（ P<0. 05）， while the miR-21 inhibition group showed increased PTEN expression（ P< 0. 05）and decreased p-AKT/AKT and p-mTOR/mTOR ratios（P<0. 05）. Compared with the miR-21 inhibition group，the miR-21 inhibition + MK-2206 group had lower p-AKT/AKT and p-mTOR/mTOR ratios（P<0. 05），with no significant difference in PTEN protein expression（P>0. 05）. HE and Masson staining showed normal kidney structure and almost no fibrosis in the control group. The model group exhibited glomerular enlargement，capillary loop adhesion，and focal fibrosis. The miR-21 overexpression group showed severe destruction of glomerular struc- ture，accompanied by extensive fibrosis and renal tubular atrophy. The pathological changes and degree of fibrosis were alleviated in the miR-21 inhibition group. The miR-21 inhibition + MK-2206 group showed only mild patho- logical changes and mild fibrosis，with the interstitium being largely normal. Compared with PTEN-WT + NC mim- ics 1，the relative luciferase activity in the PTEN-WT + miR-21 mimics group decreased（P<0. 001）. There was no statistically significant difference in relative luciferase activity between PTEN-WT + NC mimics group and PTEN-MUT + miR-21 mimics group（P>0. 05）. Conclusion miR-21 may improve renal function indicators and alleviate renal fibrosis in chronic kidney disease mice via targeted modulation of PTEN and subsequently inhibiting the AKT/mTOR pathway.