A graded index poly(methyl methacrylate) (PMMA) preform has been made by a novel interfacial-gel polymerization procedure, in which polymerization was performed in a PMMA tube and a dopant, bromobenzene (BE), was used as a higher refractive index molecule. A detailed mechanism of the graded index formation is given and simulated according to the free volume theory and its application in a high-conversion polymerization system. Under tile assumption that polymer concentration distributes in the form of omega = a exp(r) + b at the radial direction of the tube, a quadratic distribution of refractive index on the cross section of the preform is obtained, The result is identical with experiments performed by the interfacial-gel polymerization.