This paper takes the basic object of brown granulated sugar remelted syrup to introduced the purification effect and mechanism of magnesium nitrate-phosphoric acid composite detergent on the remelted syrup under the condition of high pH. It makes the option to optimize the purification condition in way of orthogonal test on the basis of the single factor test to determine the element and level of the orthogonal test. The optimum conditions are like this: 2% magnesium ion and 1.70% phosphoric acid composite detergent dosage 2%, pH value 11.0, PAM dosage 2 mg?L-1, temperature 30 ?C, these conditions registers 88.2% of the decolorization rate and 69s of the best falling time. The equipment of SEM, TEM, Nano-ZS90 ZETA potential analyzer, and XRD are harnessed to characterize the complexes in the sucrose system and the brown granulated sugar remelted syrup system and to discuss the related mechanism.