Diabetic nephropathy (DN) is often accompanied by mesangial cell proliferation and fibrosis. Tanshinone IIA (Tan-IIA) is the main fat-soluble component of Salvia miltiorrhiza. N6-Methyladenosine (mA) modification is a widely studied epigenetic mechanism. This study aimed to investigate the role of Tan-IIA in DN and the underlying mechanism. Cell viability and proliferation were assessed via cell counting kit-8 and ethynyldeoxyuridine assays. Protein levels of fibrosis-related indicators were detected by Western blot. Reverse transcription-quantitative polymerase chain reaction was used to detect the levels of mA-related enzymes. The interaction betweenWT1 associated protein (WTAP) and prolactin receptor (PRLR) was examined through RNA immunoprecipitation and dual-luciferase reporter assays. The animal DN models was established. Biochemical measurements of rat serum were performed using commercial kits. Hematoxylin&eosin and Masson trichrome staining were used for histopathological analysis. Results showed that Tan IIA treatment inhibited the cell proliferation and fibrosis of human renal mesangial cells (HRMCs). Besides, Tan IIA treatment regulated WTAP-mediated mA modification. Overexpression of WTAP upregulated the cell proliferation and fibrosis of HRMCs. Mechanically, WTAP enhanced the stability of PRLR mRNA via mA methylation. Subsequent rescue investigations revealed that overexpression of PRLR increased the cell proliferation and fibrosis of HRMCs. In the in vivo study, Tan IIA treatment reversed the renal injury in rats and decreased the protein levels of WTAP, PLRP, and fibrosis-related indicators in kidney tissues. Tan IIA suppressed the proliferation and fibrosis of HRMCs in DN though WTAP-mediated mA methylation of PRLR.