A method for preparing low-cost clean steel, comprising steps of: 1) preliminarily desulfurizing iron melt: preliminarily desulfurizing in an iron melt channel during blast furnace tapping and during iron folding in an iron folding room, adding a desulfurizing ball into the iron melt during the blast furnace tapping or the iron folding, in such a manner that after preliminarily desulfurizing, S ≤ 0.01% by weight in the iron melt before being sent into a converter; 2) desulfurizing during pre-processing of the iron melt: finely desulfurizing the iron melt by dusting desulfurization, and filtering out desulfurized slags by a slag filter, in such a manner that S ≤ 0.0015% by weight in the iron melt after the fine desulfurizing and before the iron melt is sent into a converter; 3) dephosphorizing and controlling sulfur: dephosphorizing and controlling sulfur during converter steelmaking, in such a manner that P ≤ 0.014% and S ≤ 0.004% during tapping; 4) rapidly dephosphorizing by slag-forming: rapidly dephosphorizing by slag-forming during converter tapping; at a converter end point, controlling a C content at 0.02~0.10%, controlling an oxygen activity value α O at 600~1000ppm, adding a dephosphorizing ball through an alloy chute during the converter tapping, blowing argon and stirring at the same time; 5) purifying steel melt during RH refining: adding a purifying ball at a late stage of the RH refining when a vacuum degree is at 66.7~500Pa; and 6) continuously casting with whole-process protection; wherein the desulfurizing ball comprises: white slags cool-collected by a ladle furnace 20~55%, CaO 20~50%, CaF 2 5~15%, and CaCO 3 5~15% by weight, wherein particle sizes of the CaO, CaF 2, CaCO 3 and the white slags cool-collected by the ladle furnace are less than 100μm; wherein the dephosphorizing ball comprises: white slags cool-collected by a ladle furnace 10-65%, CaO 10~65%, CaF 2 1~15%, and CaCO 3 5-30% by weight, particle sizes of the CaO, CaF 2, CaCO 3 and the white slags cool-collected by the ladle furnace are less than 100μm; and wherein the purifying ball comprises: white slags cool-collected by a ladle furnace 10-60%, CaO 15~65%, CaF 2 1~15%, CaCO 3 5-30%, and Ca powder 1~15% by weight, particle sizes of the CaO, CaF 2, CaCO 3 and the white slags cool-collected by the ladle furnace are less than 100μm. 1) preliminarily desulfurizing iron melt: preliminarily desulfurizing in an iron melt channel during blast furnace tapping and during iron folding in an iron folding room, adding a desulfurizing ball into the iron melt during the blast furnace tapping or the iron folding, in such a manner that after preliminarily desulfurizing, S ≤ 0.01% by weight in the iron melt before being sent into a converter; 2) desulfurizing during pre-processing of the iron melt: finely desulfurizing the iron melt by dusting desulfurization, and filtering out desulfurized slags by a slag filter, in such a manner that S ≤ 0.0015% by weight in the iron melt after the fine desulfurizing and before the iron melt is sent into a converter; 3) dephosphorizing and controlling sulfur: dephosphorizing and controlling sulfur during converter steelmaking, in such a manner that P ≤ 0.014% and S ≤ 0.004% during tapping; 4) rapidly dephosphorizing by slag-forming: rapidly dephosphorizing by slag-forming during converter tapping; at a converter end point, controlling a C content at 0.02~0.10%, controlling an oxygen activity value α O at 600~1000ppm, adding a dephosphorizing ball through an alloy chute during the converter tapping, blowing argon and stirring at the same time; 5) purifying steel melt during RH refining: adding a purifying ball at a late stage of the RH refining when a vacuum degree is at 66.7~500Pa; and 6) continuously casting with whole-process protection; wherein the desulfurizing ball comprises: white slags cool-collected by a ladle furnace 20~55%, CaO 20~50%, CaF 2 5~15%, and CaCO 3 5~15% by weight, wherein particle sizes of the CaO, CaF 2, CaCO 3 and the white slags cool-collected by the ladle furnace are less than 100μm; wherein the dephosphorizing ball comprises: white slags cool-collected by a ladle furnace 10-65%, CaO 10~65%, CaF 2 1~15%, and CaCO 3 5-30% by weight, particle sizes of the CaO, CaF 2, CaCO 3 and the white slags cool-collected by the ladle furnace are less than 100μm; and wherein the purifying ball comprises: white slags cool-collected by a ladle furnace 10-60%, CaO 15~65%, CaF 2 1~15%, CaCO 3 5-30%, and Ca powder 1~15% by weight, particle sizes of the CaO, CaF 2, CaCO 3 and the white slags cool-collected by the ladle furnace are less than 100μm.