Objective To investigate the expression and clinicopathological significance of carboxypeptidase A4 (CPA4) in triple negative breast cancer (TNBC).Methods The expression of CPA4, aldehyde dehydrogenase 1, NANOG, E-cadherin and Vimentin in TNBC tissues were detected by immunohistochemistry. The relationship between CPA4 expression and clinicopathological indicators in 168 cases of TNBC was analyzed. CPA4 was knocked out in MDA-MB-231 cells to observe the effects on cell proliferation, invasion and migration. The expression of related proteins was detected by Western Blot.Results The expression rate of CPA4 in TNBC tissues was 57.14%, significantly higher than that in non-TNBC tissues (37.5%), the difference was statistically significant (χ2=5.009, P=0.025). CPA4 expression in TNBC was significantly higher than that in breast hyperplasia tissues by 20%, and the difference was statistically significant (χ2=9.850, P=0.002). The expression of CPA4 in 168 TNBC patients was not associated with ALDH1 (χ2=1.575, P=0.209), but with NANOG (χ2=4.205, P=0.040), E-cadherin (χ2=11.764, P=0.001), Vimentin (χ2=4.797, P=0.029), EGFR (χ2=4.057, P=0.044). SiCPA4 inhibits TNBC clone formation, migration and invasion, inhibits the expression of ALDH-1, NANOG, vimentin and promotes the expression of E-cadherin.Conclusion CPA4 might play a role in the stem progression of TNBC and the transformation of EMT, and CPA4 might be an important therapeutic target for TNBC invasion and metastasis phenotypes.