The invention discloses a preparation method of a water-quenched manganese slag insulating wall material, which comprises the following steps: putting water-quenched manganese slag into an electric heat blasting constant-temperature drying box of 110 DEG C, drying the water-quenched manganese slag until the water content is less than 1 percent, and filtering with a 5mm square-hole sieve, thereby obtaining crude water-quenched manganese slag; milling for 5 minutes, taking out, and filtering with a 80mum square-hole sieve until the screened residue accounts for 58 percent as a reference index, thereby obtaining fine water-quenched manganese slag; mixing cement of 147-163kg/m3 and the fine water-quenched manganese slag of 360-380kg/m3, adding water of 360-370kg/m3 and the crude water-quenched manganese slag of 710-750kg/m3, and stirring for 5-8 minutes, until uniform mixture is obtained; adding the mixture into a test die, vibrating twice to three times on a vibration table, curing for 3hours at the room temperature, adding into an accelerated curing box for 18-hour curing, stripping the die 1 hour after being taken out, and curing for 7 days in water. The wall material prepared in the invention has the advantages of light weight, thermal insulation and heat isolation.