A neutron capture therapy system, including a beam shaping assembly and a vacuum tube. The beam shaping assembly includes a beam entrance, an accommodating cavity for accommodating the vacuum tube, a moderator adjacent to an end of the accommodating cavity, a reflector surrounding the moderator, a radiation shield disposed in the beam shaping assembly, and a beam exit. A target is disposed at an end of the vacuum tube, nuclear reactions occur between the target and a charged particle beam entering through the beam entrance to generate neutrons. The moderator moderates the neutrons, the reflector guides deflected neutrons back to the moderator. The moderator at least includes two cylindrical moderating members with different outer diameters respectively, the moderator has a first end close to the beam entrance and a second end close to the beam exit, and the target is accommodated between the first end and the second end.
