The invention discloses a method for preparing nanometer magnesium oxide and active light calcium carbonate, aiming to solve the problems of expensive equipment, high energy consumption, material producing area limit, and the like in the prior art. The method for preparing the nanometer magnesium oxide comprises the following steps: smashing dolomites; calcining the dolomites at low temperature; ball-milling and slaking a mixture; primary carbonizing; primary filtering; pyrolyzing; secondary filtering; mechanically ball-milling; and calcining. The method for preparing the active light calcium carbonate comprises the following steps: smashing the dolomites; calcining the dolomites at low temperature; ball-milling and slaking; primary carbonization; primary filtering; pulping filter residues and secondary carbonizing; filtering and washing the filter residues; pulping and activating the filter residues; filtering; and drying. CO2 generated by calcining the dolomites at the low temperature is delivered to the primary carbonization and the secondary carbonization to be used; secondary filtering liquid containing magnesium, which is obtained by filtering pyrolyzed liquid, is delivered to the ball-milling and slaking process or the primary carbonization process of the mixture to be continuously used; and the filtering liquid containing the magnesium and hot water washing liquid are delivered to the pyrolyzing process of the method for preparing the nanometer magnesium oxide to be continuously used.