The preparation method for graded index polymer optical fiber was analyzed using co-extrusion methods.According to the calculation theory about two refractive index and the function of refractive index distribution of the GI-POF,the dopants′ mass was calculated.Based on the unstable diffusion theory and finite difference method,the forming of refractive index distribution was described and the mathematical model was obtained.The general approximate way to describe the forming of refractive index distribution of the GI-POF was attained.