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In semi-steelmaking, the bottom-blown O2–CaO process is employed to achieve high efficiency dephosphorization. However, there is little research on the requirements for smelting parameters. In this paper, the mechanism changes of temperature effect on dephosphorization are studied by the industrial tests in a 300 t dephosphorization converter. Firstly, the influences of temperature on the equilibrium constant (KP), CaO melting ratio (K) and slag viscosity (ƞ) were analyzed. KP decreases, K increases and ƞ decreases to stability with increasing temperature. The dephosphorization effect is better than the conventional process' even at higher ƞ, because the CaO powder has melted and P is mostly removed during the process of fire-spot zone upward floatation. Hence, the requirements for slagging with good fluidity are reduced. Then, the relationships between the three factors and phosphorus partition between slag and hot metal (LP) were investigated by the correlation analysis. The importance order in the bottom-blown O2–CaO process is: lgKP＞η, it demonstrates that the melting of CaO powder doesn't affect dephosphorization. While the influence order in the conventional process is: K＞η＞lgKP. Finally, we summarize the dephosphorization reaction in the bottom-blown O2–CaO process includes transient reaction accounts for approximately 86% and continuous reaction with about 14%. The process of CaO upward floatation plays major roles in dephosphorization. Based on above results, the temperature requirement of molten steel for dephosphorization becomes lower. The industrial tests show the optimum temperature range for dephosphorization decreases from 1593-1603 K to 1573–1583 K, which validate the conclusion.