The invention relates to the technical field of filter materials, and in particular, relates to a preparation method of a light biological filter material. The biological filter material is prepared from the following components in parts by mass: 60-80 parts of steel slag, 12-18 parts of humic acid, 2-8 parts of ammonium carbonate, 0.6-0.8 part of an adhesive and 10-30 parts of water. The preparation method comprises the following steps: (1) weighing steel slag, humic acid, ammonium carbonate and the adhesive in proportion, adding water, uniformly mixing, preparing into balls in a ball formingmill, transferring into a drying oven, drying, calcining, and cooling to obtain a semi-finished product; (2) soaking the biological filter material semi-finished product obtained in the step (1) in anutrient solution for 30-45 min, and carrying out microbial inoculation for 5-7 d, to obtain the biofilm-culturing biological filter material. According to the prepared light biological filter material, the sewage treatment efficiency per unit volume is improved by 7-8 times compared with that of a conventional filter material, the environment-friendly emission standard can be met, the oxygenating efficiency is high, energy consumption is reduced, and the problem that the treatment efficiency is reduced due to the fact that the material surface is not uniform after an existing filter materialruns for a period of time is solved.