A simulative industrial processing, slag refining under open air atmosphere conditions, has been established to observe the removal efficiency of impurities from molten Si at 1823 K. The behavior of Ca and its compounds in Si are investigated. The results show that, CaSi2 is a key compound on removing Fe, Al, Ca and P from Si. Ca in molten Si is produced from the reaction between CaO and Si at the melting stage of slag and Si, as well as at the beginning refining period of around 10 min, which is largely depends on the net content of single CaO phase. With the temperature going down to 1303 K gradually, most of Si is solidified and Ca percentage is increased, then species CaSi2 phase is generated from the melt composed of 61.4 wt% Si, while Fe is precipitated at the boundary between phases CaSi2 and Si. Furthermore, it is determined that, part of P in Si is oxidized to PO43- not by SiO2 but by CaO to form CaO center dot P2O5 slag phase, the other is more likely to be reduced to CaxSiyPz than to Ca3P2. Therefore, a slag rich in CaO will be much easier to produce Ca in Si and more efficient to remove impurities by acid leaching.