In the Monte Carlo Calculation，we simulate the neutron tracks or histories by means of the random nature of the neutron interactions with the medium of shield which is a shell of the sphere made of Iron，and neutrons are produced by the T（d ，n)⁴ He reaction with 300 keV deuteron beam energy. By the explanations and analyses of results based on the Monte Carlo Calculation, we obtain the optimal thickness of shield, which slows down neutrons from 14 MeV to 1.0 MeV. The calculated results agree with those of the same kind shields of foreign countries, and have certain value of reference in applied field.