In this paper, PMMA-BB graded-index polymer optical fiber (GI POF) preform was prepared by interfacial-gel polymerization technique, whose refractive index distribution was measured by reflection method. The mechanism of the preparation and the regularities of the refractive index distribution were discussed. Gel effect and selective diffusion is considered to be the main reason that results in forming a parabolic refractive index distributing of GI POF preform, and the polymerization rate is considered to be the key role to form the optimum refractive index distribution of GI POF preform.