Abstract: In order to enhance slag detachability of agglomerated flux for low-alloy steel in root bead,uniform design method was used to optimize slag systems for MgO-Al₂O₃-CaO with high basicity and slag detachability test.Then scanning electron microscopy (SEM),energy dispersive x-ray analysis (EDAX) and X-ray diffraction (XRD) were used to analyze the microstructure,compositions and phase of slag.The results show that the No.10 slag with higher detachability is mainly made of compound rock phases,whose main elements are Zr,Mg,Ca, Al and Si.Since the element content is different in slag microzone, it can form snowflake grain with Zr,cross grain with Mg, and dentrite with Ca.Dentrite with Ca and snowflake grain with Zr can hinder the growth of cross grain with Mg.In addition,Zr has a role in refining cross grain with Mg.So the increase of the contents of marble and zircon sand in flux can change the continuity and direction of cross grain with Mg in slag.It will be a valid way to improve slag detachability.