The mixed dibasic acids (DBA) is a by-product during adipic acid (ADA) production by oxidation of cyclohexanol or cyclohexanone, which contains large amount of glutaric acid (GA) with ADA and succinic acid (SA). In this work, we have explored the feasibility of the coupled solution crystallization and melt crystallization for the purification of GA. To obtain fundamental thermodynamic properties for the process design, the gravimetric method was used to measure the solubility of the three dibasic acids in water from 293.15 K to 345.15 K at atmospheric pressure. The results showed that GA was the most soluble component while all solubility increased with increasing temperature. Therefore, it is reasonable to concentrate GA in the liquid phase by cooling crystallization. Similarly, the ternary solid-liquid melt equilibrium phase diagram was constructed based on the experimental data of the three binary subsystems (GA-ADA, GA-SA, ADA-SA). It was found that the concentrated GA after solvent evaporation could be purified by melt crystallization efficiently. Thus a coupled solution crystallization and melt crystallization process was designed. With the operated operating parameters, the purity of GA could reach 99.06% by secondary melt crystallization. Overall, the coupled solution crystallization and melt crystallization method to separate DBA has proven to be effective.