To improve coupling efficiency from blackbody source into fiber, a coupling model of light intensity from the blackbody cavity into a spherical optical fiber was proposed. Influence of parameters of spherical fiber and blackbody cavity on coupling efficiency was analyzed. Results show that coupling efficiency firstly increases and then decreases with the increase of radius of spherical fiber. When radius is around 85 μm, coupling efficiency reaches the maximum. Spherical fibers with radius from 65 μm to 105 μm were fabricated by fusion splicer. Experiments were carried out with spherical fibers and a fiber with flat end face. Results show that compared with the fiber with flat end face, coupling efficiency of spherical fiber is greatly improved, when radius is around 85 μm, coupling efficiency is improved by approximately 60%. The experimental results are consistent with the theoretical analysis.