Abstract: In boron neutron capture therapy (BNCT), beam shaping assembly is one of the key components in BNCT device to produce high quality neutron beams, and its design is of great importance. Based on the process of 2.5 MeV proton hitting lithium target to produce neutrons, the beam shaping assembly of accelerator driven BNCT neutron source was designed. Then the influences of moderator thickness difference on exit beam quality, dose distribution in head phantom and clinical parameters were studied. The results show that the feasibility design can meet the requirements of IAEA for beam quality under the driving of 30 mA proton beam. Under the three designs of the thickness of the moderator in this paper, with the increase of moderator thickness, the intensity of exit epithermal neutron beam and the fast neutron fraction decrease, which further leads to the shallower treatment depth, the lower maximum dose rate of normal tissue and the longer treatment time.