The invention discloses a process method for treating cobalt slag of formazan, which comprises the following steps: (1), acid washing and zinc removal, (2), roasting, (3), water immersion, (4), extraction and purification, ( 5) Precipitation of ammonium oxalate, (6) Calcination of cobalt oxalate, the calcination includes adding 10-30wt% sodium persulfate to the pickling slag obtained by pickling and dezincification, after one stage of calcination, the temperature is raised for a second stage of calcination. The beneficial effect of the invention is that in the pickling process, most of the zinc can be recovered by pickling under normal pressure and low pressure, without using high-temperature pressure leaching and other strengthening means, which reduces equipment investment and production costs. Sodium sulfate is used as a roasting additive. It decomposes to generate oxygen to provide an enhanced oxidizing atmosphere for thermal decomposition. The generated sodium sulfate replaces the commonly used concentrated sulfuric acid, reduces the generation of acid mist, and can also destroy the dense oxide film formed on the surface of the ore particles, so that the oxidation of the inner layer Cobalt continues to contact with O2, SO2, SO3, so as to achieve complete sulfation and improve the recovery rate of cobalt.