The invention relates to a slag-corrosion-resistant lightweight alumina clinker and a preparation method thereof. According to a technical scheme in the invention, the preparation method comprises thefollowing steps: with 89-97 wt% of raw alumina micropowder, 1-9 wt% of carbon black and 0.1-4 wt% of plant ash as raw materials, uniformly mixing the raw materials in a planetary ball mill, and carrying out mechanical pressing under the condition of a pressure of 100-200 MPa to obtain a green body; and drying the green body at 110-200 DEG C for 12-36 hours, and carrying out heat preservation at 1550-1750 DEG C under a carbon burying condition for 3-8 hours to obtain the slag-corrosion-resistant lightweight alumina clinker. The method is low in cost and simple in process, and the prepared slag-corrosion-resistant light-weight alumina clinker has the characteristics of small pore diameter, high strength and strong slag resistance.