The invention designs a filtering precipitating method of a dual-alkali method based desulfurization system; the system comprises a regenerating pool, a precipitating pool, a circulating pool, a stirring device, a bag filter, an air compressor and an air compressor pipeline. Calcium hydroxide slurry in a calcium oxide dissolving tank and outgoing desulfurized liquid from a desulfurizing column enter the regenerating pool for reaction, the regenerating pool is communicated with the precipitating pool, the stirring device is arranged in the precipitating pool, the precipitating pool is connected with the circulating pool through the bag filter, the air compressor is arranged on one side of the circulating pool, the air compressor is connected with a compressed air pipeline, the other end of the compressed air pipeline is inserted into the bag filter, and the air compressor introduces air periodically into the bag filter through an automatic control system; clear liquid from the circulating pool is pumped into the desulfurizing column through a circulating pump for reaction. The filtering precipitating method has the advantages that precipitate can be well avoided entering the circulating pool, blockage of filter cloth by the precipitate due to long-term usage is effectively avoided, a filtering mode facilitating replacement and having low manufacture cost is provided, energy can be saved, and the efficiency of the desulfurization system can be improved.