The present invention discloses a method for synergistically purifying SO2 and NOx in tin smelting flue gas by smelting furnace slag. Desulfurization efficiency can reach 95.0% and denitration efficiency can reach 58.6%. In the method, firstly, slag slurry absorbing liquid is used to conduct countercurrent contact reaction with flue gas; transition metal ions of iron, manganese, etc. in slag slurry conduct catalytic oxidation reaction with SO2 and NO in the flue gas to realize simultaneous removal of the SO2 and NOx; the SO2 in the flue gas is absorbed and converted into sulfuric acid which isused as a valuable metal leaching agent in furnace slag, so that treatment of wastes with processes of wastes against one another is realized; the periodically discarded circulating absorption slurrycan leach valuable metals by a wet processing technology to realize resource utilization. The method is suitable for operating conditions of relatively wide temperature and gas flow rate ranges, etc.The method is also suitable for purification treatment of other low-SO2-concentration smelting tail gas in addition to the synergistic purification treatment of the tin smelting flue gas.