Based on convection-diffusion effect, a fluid lens with optical gradient refractive index microcavity was established by using finite element method. The stable concentration distribution of the core and cladding liquid after convenction-diffusion process was analyzed, which was called refractive index distribution of mixed fluid. The structure of convection-diffusion model was studied. For gradient refraction index characteristics of optical microfluidic tunable lens, the effects of different cross-sections on the stability of refractive index distribution along the direction of fluid flow direction were studied respectively in general(the refractive index of core fluid was larger than that of cladding fluid) and in the case of central depression(the refractive index of core fluid was smaller than that of cladding fluid).