Induced crystallization step-by-step softening method and system

The invention provides an induced crystallization step-by-step softening method and system. The system comprises an adjusting tank (1), a first-stage granulation system (2), a heat exchange system (3), a second-stage granulation system (4), a security filter device (5) and a wastewater decrement concentration system (6) which are connected in sequence, wherein the first-stage granulation system (2) and the second-stage granulation system (4) are both circulating fluidized-bed devices. According to the method, high-magnesium desulfurization wastewater is softened step by step through the two-stage series induced crystallization granulation system, CaCO3 crystals are recycled firstly, then flue gas or waste steam in a plant is used for heating the high-magnesium desulfurization wastewater, and softening is further conducted so as to obtain Mg(OH)2 crystals with high commodity value. According to the method and the system, the high-magnesium desulfurization wastewater can be softened, the CaCO3 crystals and the