α_S1-Casein, encoded by the CSN1S1 gene, is one of the main milk proteins, which can provide amino acids and other nutrients for the bodies, nevertheless, it can also lead to human allergy. However, the effects of CSN1S1 on other milk proteins in goat mammary epithelial cells (GMEC) remain unknown. To explore the regulatory mechanism of CSN1S1 on major milk proteins, CSN1S1 overexpression adenovirus and small interference RNA targeting CSN1S1 were designed and transfected into GMEC, respectively. GMEC were incubated with treatments for 48 h, and then total RNA and protein of cells were isolated. The mRNA levels and protein levels of caseins (CSN1S1, CSN1S2, CSN2, CSN3), major whey proteins (LALBA, BLG), proteins of Janus kinase 2/ signal transducer and activator of transcription 5a (JAK2/STAT5a) signaling pathway (JAK2, STAT5a, SOCS3, ELF5, PRLR) were detected by RT-qPCR and Western blotting, and analyzed with Student’s t-tests. Results showed that overexpressing CSN1S1 decreased the β-casein mRNA and protein abundance, as expected, CSN1S1 silencing resulted in a significant increase in β-casein (P < 0.05). However, CSN1S1 did not alter the expression of CSN1S2, CSN3, LALBA, BLG (P > 0.05). Previous studies have shown that JAK2/STAT5a signaling promotes β-casein transcription and synthesis in ruminants, which implied the possibly involvement of JAK2/STAT5a signaling pathway in CSN1S1 regulating β-casein metabolism. We observed that overexpression of CSN1S1 induced reductions of JAK2 and STAT5a mRNA level, as well as STAT5a phosphorylation level (P < 0.05). Similarly, knockdown of CSN1S1 upregulated the mRNA level of JAK2 and STAT5a, also increased phosphorylation abundance of STAT5a (P < 0.05). In conclusion, our findings indicated the regulatory role of CSN1S1 on β-casein through JAK2/STAT5a signaling pathway in goat mammary epithelial cells.