In present work, the role of the oxygen potential (P-O2) and oxygen ion (O2-) concentration for removing phosphorus (P) during CaO-SiO2-Al2O3-FexO slag refining was studied by on-line measurement of oxygen activity in molten silicon (Si), FactSage calculation, Raman spectroscopy and nuclear magnetic resonance (NMR) spectroscopy. The results show that the addition of FeO from 0 to 9.25 wt% in slag can increase the activity of dissolved oxygen (a([O])) in Si and the mole fraction of O2- in slag. Moreover, the increase of O2- concentration leads to the increase of non-bridge oxygen (NBO). The value of L-P (the partition ratio of phosphorous between slag and Si shows a first increase and then decrease trend and reaches a maximum value of 1.95 at 5 +/- 0.1 wt% FeO. It is believed that the increase of a([O]) and NBO can promote the removal of P as FeO content is less than 5 +/- 0.1 wt%. the chain structure unit (Q(2)) of silicate network as the main intermediate structure to capture PO43- from the charge compensation of P2O5 by O2- to form the sheet structure unit Q(3)(Si and P). When FeO content is increased to more than 5 +/- 0.1 wt%, L-P value gradually decreases although the values of NBO and a([O]) are increasing. NBO plays a leading role in this process, it can be speculated that more NBO can depolymerize the Q(3) (Si and P) to destroy the stability of P in silicate network. As a result, a mount of PO43- is present at the interface to prevent the oxidation of phosphorous, which leads to the decrease of L-P value.