The invention discloses a multi-stage absorbing desulfurizing device by adopting an ammonia-ammonium sulfate method. The absorbing desulfurizing device comprises a flue gas desulfurizing absorption tower, an absorption liquid circulating tank, a thin slurry circulating tank and a wet-type electric dust remover, wherein the flue gas desulfurizing absorption tower comprises a concentrated slurry circulating tank, a concentration pre-absorption section, an absorption section, a thin slurry washing section and a defogging section that are arranged in a vertical manner; the concentration pre-absorption section comprises at least a layer of concentrated slurry spraying layer; the absorption section comprises an absorption liquid cyclone defogging liquid collector and at least a layer of absorption liquid spraying layer; the thin slurry washing section comprises a thin slurry cyclone defogging liquid collector and a thin slurry spraying layer; the defogging section comprises a defogger and a defogger washing spraying layer. According to the multi-stage absorbing desulfurizing device by adopting the ammonia-ammonium sulfate method, provided by the invention, flue gas cooling, multi-stage defogging absorption and multi-section oxidization are completed in the desulfurizing absorption tower, the utilization rate of ammonia is increased by regulating the ammonia adding positions, and the problems that the concentration of slurry in multiple sections is controlled unsatisfactorily, and the content of aerosol in flue gas after desulfurization is too high are solved, wherein the problems are caused due to the fact that although the conventional desulfurizing absorption tower is divided into multiple sections, slurry in the multiple sections are carried with gases heavily.