A neutron capture therapy system , comprising: an accelerator configured to generate a charged particle beam (P); a neutron generator configured to react with the charged particle beam to generate neutrons, the neutrons forming a neutron beam (N), the neutron beam defining a main axis (I); a vacuum tube connected to the accelerator; and a beam shaping assembly which comprises, an accommodating portion ; a moderator adjacent to the neutron generator to moderate the neutrons generated by the neutron generator to an epithermal neutron energy region; a reflector surrounding the moderator to guide deflected neutrons back to the moderator to increase the intensity of the epithermal neutrons; a radiation shield configured to shield leaked neutrons and photons to reduce dosage to normal tissue in a non-irradiation area; and a beam exit disposed in the beam shaping assembly, wherein the vacuum tube is provided at the accommodating portion, the neutron generator is disposed at an end of the vacuum tube, and the vacuum tube is configured for transmitting charged particles (P) accelerated by the accelerator to the neutron generator, wherein the neutron generator is configured to move between a first position where the neutron generator reacts with the charged particle beam to generate neutrons and a second position where the neutron generator separates from the beam shaping assembly, characterized in that : a thermal neutron absorber is disposed adjacent to the moderator; and an adjustment of an overall length of the vacuum tube provides a space for the neutron generator to move from the first position to the second position. an accelerator configured to generate a charged particle beam (P); a neutron generator configured to react with the charged particle beam to generate neutrons, the neutrons forming a neutron beam (N), the neutron beam defining a main axis (I); a vacuum tube connected to the accelerator; and a beam shaping assembly which comprises, an accommodating portion ; a moderator adjacent to the neutron generator to moderate the neutrons generated by the neutron generator to an epithermal neutron energy region; a reflector surrounding the moderator to guide deflected neutrons back to the moderator to increase the intensity of the epithermal neutrons; a radiation shield configured to shield leaked neutrons and photons to reduce dosage to normal tissue in a non-irradiation area; and a beam exit disposed in the beam shaping assembly, wherein the vacuum tube is provided at the accommodating portion, the neutron generator is disposed at an end of the vacuum tube, and the vacuum tube is configured for transmitting charged particles (P) accelerated by the accelerator to the neutron generator, wherein the neutron generator is configured to move between a first position where the neutron generator reacts with the charged particle beam to generate neutrons and a second position where the neutron generator separates from the beam shaping assembly, characterized in that : a thermal neutron absorber is disposed adjacent to the moderator; and an adjustment of an overall length of the vacuum tube provides a space for the neutron generator to move from the first position to the second position. a thermal neutron absorber is disposed adjacent to the moderator; and an adjustment of an overall length of the vacuum tube provides a space for the neutron generator to move from the first position to the second position. <EOS>