Although noble metal electrocatalysts are highly efficient in the dehalogenation of halogenated antibiotics, the prohibitive cost hinders their practical applications. In this study, a cobalt-phosphorous/oxide (Co-P/O) composite prepared via a one-step electrodeposition was for the first time applied in electroreductive de chlorination of halogenated antibiotics (HA), including chloramphenicol (CAP), fiorfenicol (FLO) and thiamphenicol (TAP). Co-P/O had a higher FLO dechlorination efficiency (91%) than Pd/C (69.3%) (t = 60 min, C-0 = 20 mg L-1, applied voltage of-1.2 V vs. saturated calomel electrode (SCE)). Furthermore, the de chlorination efficiencies of Co-P/O for CAP and TAP reached to 98.7 and 74.2%, respectively. The electron spin resonance and in situ Raman characterizations confirmed that atomic H-star was produced via the Co-P and the formation of Co-Cl bonds occurred on the Co-O in Co-P/O. The Co-Cl bond formation could trap HA molecules onto Co-P/O and weaken the C-Cl bond strength. The synergistic effect of H-star attack and Co-Cl bond was responsible for the high dechlorination efficiency. This study offers new insights into the interface mechanism of electroreductive dehalogenation process, and shows a great potential for the remediation of halogenated antibiotics contaminated wastewater.