A method for treating arsenic-containing flue gas is disclosed. In the method, the arsenic-containing flue gas is subjected to a dry pre-dedusting treatment, and the dedusted flue gas is pre-cooled and then introduced into a vortex quenching system. The arsenic-containing flue gas is divided into high-temperature flue gas and low-temperature flue gas through the vortex quenching system. The outlet temperature of the low-temperature flue gas is dropped below the desublimation temperature of gaseous arsenic trioxide. The low-temperature flue gas is subjected to a gas-solid separation to obtain solid arsenic trioxide and treated flue gas.