Objective To observe the effect of miR-21-3p on mice with renal ischemia / reperfusion （I/R） injury and to explore whether the renal protective effect is related to the regulation of apoptosis levels through miR-21-3p/p53 signaling pathway. Methods In vivo experiments were randomly divided into five groups： Sham， I/R， I/R + agomir， I/R + agomir + PIF， and I/R + PIF. Renal I/R injury model in vivo was established， overexpressing of miR-21-3p （agomiR-21-3p） in mice were constructed and treated with the p53 inhibitor Pifithrin-α （PIF）. Dual-luciferase reporter assay was used to verify the interaction between miR-21-3p and p53. Serum levels of cystain C （Cys C）， blood urea nitrogen （BUN） and serum creatinine （Scr） were measured to reflect renal function of mice. Serum oxidative stress and inflammation indices were also determined， including tumor necrosis factor-alpha （TNF-α）， Interleukin-1β （IL-1β）， and high-sensitive c-reactive protein （hs-CRP）. In addition， apoptosis rate of renal cells was visualized by TUNEL staining and the expression of apoptosis b-cell lymphoma-2 （Bcl-2）， Bcl-2 assaciated X protein （Bax）， p53 and caspase-3 were measured too. Results Compared with Sham group， the expression of miR-21-3p in I/R mice was significantly decreased. Compared with I/R group， the expression of miR-21-3p in the I/R + agomir group， the I/R + PIF group and the I/R + agomir + PIF group was significantly increased （P < 0.05）， while the trend of p53 expression was opposite. Dual-luciferase reporter assay confirmed the interaction between miR-21-3p and p53. Compared with Sham group， the rate of cell apoptosis in all other renal tissues was increased； however， compared with the I/R group， the I/R + agomir， the I/R + PIF group and I/R + agomir + PIF groups were decreased， and the I/R + agomir + PIF group was the lowest （P < 0.05）. Conclusions miR-21-3p could inhibit the development of apoptosis by targeting p53 expression， thus alleviating renal injury in mice with renal I/R.