MCA/CD-PA6 composites were synthesized by the two-step-melt polymerization, in which firstly cationic dyeable polyamide 6(CD-PA6) was prepared with sodium salt from 5-sulfoisphthalic acid (5-SSIPA), and then melamine cyanurate (MCA), as efficient flame retardant, was blended into the reaction. The results indicate that SEM, FT-IR, XRD, and TGA were applied to prove the dispersion of 5-SSIPA and MCA in the polyamide 6 matrix. Incorporating 5-SSIPA creates more amorphous regions and the dyeing rate rises up easily, the highest rate is 96.79% and the color is bright. The flame retardant test show that with appropriate addition of the MCA content, the limit oxygen index reaches up to 27, smoked less, generated no more black smoke, droplet phenomenon is slightly, and the flame resistance is greatly improved.