Objective To investigate the role of miR-21-3p and its target protein signal transducer and activator of transcription 3 protein (STAT3) in keratinocytes during the pathogenesis of psoriasis vulgaris. Methods Immunohistochemical method was used to detect the expression of STAT3 protein in skin tissue, real-time fluorescent quantitative PCR method was used to detect the expression of miR-21-3p in skin lesions, and double luciferase reporter gene was used to detect the relationship between STAT3 and miR-21-3p. Chemical synthesis of miR-21-3p mimics and inhibitors, transient transfection of HaCaT cells, CCK8 and flow cytometry apoptosis experiments were carried out on different groups of cells to explore and analyze the main role of miR-21-3p in regulating cell phenotype. The expression of STAT3 protein in transfected cells was detected by Western blot. Results With real-time fluorescent quantitative PCR，the expression level of miR-21-3p was higher in lesions of psoriasis vulgaris than in the beside-lesional tissue and normal skin tissue, and differences was statistical significance (t=7.009，P＜0.01)Immunohistochemical results showed that the positive expression of STAT3 protein was located in the cytoplasm. The positive expression of STAT3 in skin lesions of patients with psoriasis population was 82.22%(37/45), it was higher than the rate of positive expression in the normal group. The two groups of data by chi-square test were statistically significant(F=25.742，P＜0.01) Transfected HaCaT cells with miR-21-3p mimic and miR-21-3p inhibitor. The results of CCK8 and flow cytometry showed that miR-21-3p mimics promoted the proliferation of HaCaT cells and inhibited apoptosis. MiR-21-3p inhibitor inhibits the proliferation of HaCaT cells and promotes cell apoptosis. The results of Western blot showed that after miR-21-3p was transfected into HaCaT cells, its overexpression could promote the expression of STAT3 protein, on the contrary, it decreased, and the difference was obvious(F=48.632,P＜0.01) The detection results of double luciferase reporter gene showed that the CO transformation of miR-21-3p-mimics + wt plasmid resulted in the decrease of fluorescence intensity ratio measured by Renilla luciferase(P＜0.05) Conclusion Mir-21-3p shows a positive regulatory relationship and could target STAT3. Both of them participate in and aggravate the course of psoriasis, inhibit apoptosis and regulate the proliferation of keratinocytes