The best coupling structure of the reflective-type-semicircular-optical fiber probe has been studied by using the method of numerical simulation. Distributive curve of the intensity of the reflective light on the incident plane of the light is given by computer simulation to the different radii of optical fiber cores which are separately 3μm, 5μm, 10μm, 15μm. and the radius of the optical fibers are totally 20m. The results show that the best coupling radius of the reflective type semicircular optical fiber probe satisfies the formula. ρ_max=R-0.01004a³+0.37429a²-3.38095a+16.85714(μm).