A series of graded index poly(methyl methacrylate) (PMMA) preforms with monomer/dopant = 5:1, 7:1, 10:1 have been made by an interfacial-gel polymerization procedure, in which polymerization was performed in a PMMA tube and a dopant, bromobenzene (BB), was used as a higher refractive index molecule. The graded index was formed by the diffusion of the molecules. While describing the process of interfacial-gel polymerization, the distribution of the polymer concentration at the radial direction of the tube was assumed to be omega = a(r/R)(2) + b. According to the self-diffusion approach of the free-volume theory in ternary systems and its application in a high-conversion polymerization system, a model of forming the graded index is established, and a quadratic refractive index (RI) distribution on the cross-section of the preform is obtained. Compared with the theory in a binary system, the result is more identical with experiments in both the changing trend and the value of RI. The RI distributions of preforms prepared with different MMA/BB were simulated. The effect of MMA/BB on the RI distribution was also discussed.