The invention belongs to a steel and iron smelting and continuous casting production technology, in particular to covering slag for continuously casting aluminum-free or ultralow-aluminum steel and a preparation method thereof. The method comprises the following steps: fully drying and dehydrating raw materials, weighing the materials according to weight parts, and burdening; mixing the materials, and pre-melting at the temperature of 1,300-1,400 DEG C; cooling pre-molten slag, smashing, grinding finely and pulping; drying, grinding finely and uniformly mixing; granulating by using a spray tower to prepare pre-molten hollow particle covering slag. The covering slag has the advantages that the covering slag does not contain aluminum oxide, and aluminum is not added into steel by controlling a steel-slag reaction; moreover, the covering slag has higher capability of absorbing aluminum oxide in steel, and is suitable for continuously casting aluminum-free or ultralow-aluminum steel which has restrict limit on the aluminum content of the steel; moreover, the indexes of melting point, melting speed, viscosity, crystallization temperature and the like of the covering slag are consistent with the requirements of continuous casting of the aluminum-free or ultralow-aluminum steel; the covering slag has the advantages of freeness from fluorine, environmental friendliness, wire raw material source, simple manufacturing method and remarkable economic benefit and social benefit.