The invention discloses a protective slag of a large-profile round billet continuous-casting crystallizer and a method for preparing same. The protective slag comprises the following components by weight percentage: 27-33% of SiO2, 24-30% of CaO, 1-3% of MgO, not more than 2% of Fe2O3, 5-10% of Al2O3, 4-9% of Na2O, 1-4% of F, 0.5-2.5% of MnO and 15-20% of C (solid). The components of the protective slag of the continuous casting crystallizer are selected according to the regions of fake siliceous lime in a three-element system CaO-SiO2-Al2O3; out of the components for composing the protectiveslag, the CaO is taken as a main alkaline material, the SiO2 is taken as a main acid material, the Na2O, the CaF2 and the Li2O are taken as main flux agents, and the carbon component is taken as a framework material and a fusing speed controlling agent. By organic combination of the basic materials and the flux agents, the chemical components of the protective slag are adjusted, so as to adjust the physico-chemical indexes of the protective slag, such as fusing temperature, viscosity, crystallization performance, absorbing and including capability, heat conducting capability and the like and meet the requirements of keeping normal slag consumption, thickness of liquid slag layer and reasonable heat conducting amount under a special profile condition, thus ensuring the thickness of the billet shell at the outlet end of the crystallizer and realizing non-gluing continuous casting production.