Neutrons constitute an important component of the radiation environment. Their energy distribution may span from thermal up to hundreds of MeV. This paper describes the simulation of a high sensitivity Bonner Sphere neutron detector uses a 3He counter tube inside a polyethylene moderator which is a part of a scientific research project named distributed dynamic radioactive detection imaging system belongs to The Ministry of science and technology of the PRC major instrument development special. For the purpose of the characterization of the response, a detailed model of the detector was developed by FLUKA Monte Carlo simulations. Several combinations of materials and different sphere diameters was investigated also.