Abstract: By thermodynamic calculations and analysis,the optimized slag composition(mass fraction) of ladle refining for low carbon low silicon Al-killed steel grades was optimized during compact strip production(CSP) process as CaO 50%-55%,Al₂O₃ 30%-36%,SiO₂ 1%-6%. The equilibrium liquid steel composition is as[Si]0.2%-0.4%,a_[O] lower than 4.5×10^-6,[Ca]2×10^-5-4×10^-5when dissolved Al is approximating 0.03%. The optimized slag is more favorable for deoxidation,desulfurization and desiliconization. In addition,it is also with low melting point,quite well fluidity as well as the ability to absorb inclusions.Finally,both slag-steel equilibrium experiments in laboratory scale and industrial trials in plant scale verified that optimized slag can very control the steel composition as well as inclusion composition and improve steel cleanness quite a lot. After applied the optimized slag,the ratio of cold rolled strips that are degraded due to surface quality problems decrease from 1.23% to 0.8%,and the costs of LF refining reduce by 4.30 RMB per ton Fe,namely approximating 10%.