The invention relates to the technical field of rotary kilns, in particular to a rotary kiln tail gas purifying method. According to the purifying method, an active carbon columns or a coke column isadopted to reduce sulfur dioxide and nitric oxide in tail gas to be elemental sulphur and nitrogen under the heating catalysis effect of microwave radiation, so that the elemental sulphur remains in active carbon particles to be recycled, and the nitrogen is exhausted along with the tail gas containing carbon monoxide and carbon dioxide. Pollution to the environment cannot be generated, and the defect that a traditional technology cannot deeply purify the tail gas and generates a large amount of waste water to cause control difficulty is overcome. Meanwhile, under the optimized microwave frequency, the reduction efficiency on sulfur dioxide and NOX in the rotary kiln tail gas is greatly improved.