The invention relates to a treatment method and treatment system for zero discharge and fractional crystallization of concentrated brine. The method comprises the following steps: pre-reaction, to bemore specific, converting HCO3<2-> into CO3<2-> and removing heavy metal ions; magnetic flocculation, to be more specific, flocculating the pre-reacted wastewater; precipitation, to be more specific,precipitating the flocculated wastewater; concentration, to be more specific, sequentially carrying out positive osmosis and reverse osmosis separation on the precipitated supernatant, and outputtingseparated purified water; crystallization, to be more specific, crystallizing the concentrated highly concentrated brine; crystal separation, to be more specific, carrying out solid-liquid separationon crystallization slurry generated by the crystallization, and outputting a separated crystalline matter; and magnetic seed separation, to be more specific, separating a magnetic seed, sludge and water and recycling the magnetic seed. The system comprises a reaction tank, a flocculation tank, a sedimentation tank, a concentration device, a crystallization reaction reactor, a solid-liquid separation device and the like. The system is suitable for treating desulfurization concentrated brine of catalytic cracking flue gas and other similar high salt wastewater, and can realize zero discharge ofconcentrated brine and crystallization salt productization.