Disclosed are a method of preparing conjugated diene and a device therefor. More particularly, disclosed a method of preparing conjugated diene, wherein generated gas including butadiene is cooled and then water discharged at a lower part is not directly treated as waste water and subjected to byproduct removal and steam-extraction to utilize converted steam, and an installation issue of an existing biological waste water disposal equipment due to an excessive amount of byproducts can be resolved, and a device therefor are disclosed.