The invention provides a method for preparing azodicarbonamide by recycling biurea. The purity of the biurea is increased after the biurea is recycled and subjected to the processes of pulverizing, pulping, mixing, washing, and pulping, and then the biurea is favorably oxidized again in the later period to form azodicarbonamide; in a process of chlorine introduction and oxidization, chlorine is introduced to a position below a reaction liquid level through a conduit, then the contact between the material and the chlorine is favorably, the gas flow of chlorine is controlled by stages, the amount of the introduced chlorine increases and then decreases again, then the control over the conversion rate of the biurea and the reaction heat is favorably improved, and the use ratio of the recycledbiurea in the preparation of the azodicarbonamide is increased. The drying efficiency is improved through a drying system, the integration of drying and washing is implemented through a vacuum belt filter, and the drying time is saved; a plurality of U-shaped heating tubes and V-shaped diversion plates are arranged in an air heater, so that the air is guaranteed to fully absorb the heat after passing through the heating tubes, and the fully heated air is outputted to make preparations for the later drying process.