The invention discloses lightweight fluorine-free environmental protection continuous casting protective slag for enamel steel. The continuous casting protection slag comprises the following components including, by mass 22%-38% of CaO, 21%-37% of SiO2, 0.8%-3.0% of Al2O3, 1%-7% of MgO, 5%-20% of heating agent Si- Ca, 0.7%-2% of Li2O, 2%-8% of B2O3, 5%-10% of Na2O, 5%-17% of elemental C and unavoidable impurities; and the slag must meet: [CaO]/[Sio2] is greater than or equal to 0.8 and less than or equal to 1.0, namely, the alkalinity of the protective slag is 0.8- 1.0 The ratio of the heatingagent Si-Ca is 36-40 %: 64- 60%. The protective slag of the invention can reduce the occurrence of cold junction steel on a liquid surface by adding suitable heating elements without changing the alkalinity of the protective slag after the addition, and the steel leakage can be avoided; by adjusting a flux, another one or several substances are used for replacing fluoride, so the corrosion of thefluorine to a casting machine and the impact on human body are reduced.