A beam shaping assembly (100, 200) for neutron capture therapy, comprising: a beam outlet (140, 240); a neutron generating device (110, 210) housed within the beam shaping assembly configured to generate neutrons (n), wherein the generated neutrons form a neutron beam in a direction from the neutron generating device to the beam outlet, the neutron beam defining a beam axis (X, Y); a moderator (120, 220) adjacent to the neutron generating device, configured to moderate the fast neutrons in the neutron beam to epithermal neutrons; a disturbing unit (130, 230); and a reflector (150, 250) surrounding externally the moderator and the disturbing unit, configured to reflect the neutrons deviating from the neutron beam back to the neutron beam to enhance the neutron beam intensity, and wherein the reflector is made of a material having a strong neutron reflection ability; wherein the beam shaping assembly produces gamma rays in the process of adjusting the neutron beam energy spectrum, and wherein the disturbing unit is a cylinder defining an axis and the axis of the cylinder coincides or is parallel to the beam axis, and wherein the disturbing unit (130, 230) is located between the moderator and the beam outlet for passing through the neutron beam and reducing the gamma ray content in the neutron beam passing through the beam outlet; and wherein when the neutron beam containing gamma rays passes through the disturbing unit, the disturbing unit reduces the gamma ray content in the neutron beam by absorbing gamma rays through photoelectric effect, scattering gamma rays through Compton effect, or converting gamma rays into positive and negative electron pairs through electron pair effect, respectively, and the gamma rays scattered by the disturbing unit is further attenuated by reabsorption or reflection after encountering the reflector. a beam outlet (140, 240); a neutron generating device (110, 210) housed within the beam shaping assembly configured to generate neutrons (n), wherein the generated neutrons form a neutron beam in a direction from the neutron generating device to the beam outlet, the neutron beam defining a beam axis (X, Y); a moderator (120, 220) adjacent to the neutron generating device, configured to moderate the fast neutrons in the neutron beam to epithermal neutrons; a disturbing unit (130, 230); and a reflector (150, 250) surrounding externally the moderator and the disturbing unit, configured to reflect the neutrons deviating from the neutron beam back to the neutron beam to enhance the neutron beam intensity, and wherein the reflector is made of a material having a strong neutron reflection ability; wherein the beam shaping assembly produces gamma rays in the process of adjusting the neutron beam energy spectrum, and wherein the disturbing unit is a cylinder defining an axis and the axis of the cylinder coincides or is parallel to the beam axis, and wherein the disturbing unit (130, 230) is located between the moderator and the beam outlet for passing through the neutron beam and reducing the gamma ray content in the neutron beam passing through the beam outlet; and wherein when the neutron beam containing gamma rays passes through the disturbing unit, the disturbing unit reduces the gamma ray content in the neutron beam by absorbing gamma rays through photoelectric effect, scattering gamma rays through Compton effect, or converting gamma rays into positive and negative electron pairs through electron pair effect, respectively, and the gamma rays scattered by the disturbing unit is further attenuated by reabsorption or reflection after encountering the reflector.