A radiation irradiation system comprising: a radiation irradiation device; and a treatment bed for transporting a target to be irradiated to the radiation irradiation device for irradiation, wherein the treatment bed comprises: a mounting table on which the target to be irradiated is placed; a supporting portion for supporting the mounting table; wherein the system further comprises a positioning assembly provided on the mounting table for positioning the target to be irradiated, wherein the positioning assembly comprises: a shielding body including polymer and radiation shielding material capable of shielding the radiation, wherein the shielding body is made of silicone, radiation shielding material and silicone curing agent, and is in a form of solid particles; and a sealing bag for accommodating the shielding body, wherein the radiation irradiation system is a neutron capture therapy system, the radiation irradiation device is a neutron capture therapy device including a neutron source for generating a mixed radiation field including neutrons and photons, the shielding material is neutron capture material, the neutron capture material is capable of shielding neutrons and is made of at least one of a boron-containing compound and a lithium-containing compound, wherein the positioning assembly is configured to be recessed with a shape of the target at the position where the target is placed and form a contour corresponding to the target to position the target to be irradiated upon placing the target to be irradiated on the positioning assembly and by vacuuming the sealing bag, and wherein the positioning assembly is configured to restore to its state before being vacuumed after the target to be irradiated on the surface of the sealing bag is removed and the sealing bag is filled with air. a radiation irradiation device; and a treatment bed for transporting a target to be irradiated to the radiation irradiation device for irradiation, wherein the treatment bed comprises: a mounting table on which the target to be irradiated is placed; a supporting portion for supporting the mounting table; wherein the system further comprises a positioning assembly provided on the mounting table for positioning the target to be irradiated, wherein the positioning assembly comprises: a shielding body including polymer and radiation shielding material capable of shielding the radiation, wherein the shielding body is made of silicone, radiation shielding material and silicone curing agent, and is in a form of solid particles; and a sealing bag for accommodating the shielding body, wherein the radiation irradiation system is a neutron capture therapy system, the radiation irradiation device is a neutron capture therapy device including a neutron source for generating a mixed radiation field including neutrons and photons, the shielding material is neutron capture material, the neutron capture material is capable of shielding neutrons and is made of at least one of a boron-containing compound and a lithium-containing compound, wherein the positioning assembly is configured to be recessed with a shape of the target at the position where the target is placed and form a contour corresponding to the target to position the target to be irradiated upon placing the target to be irradiated on the positioning assembly and by vacuuming the sealing bag, and wherein the positioning assembly is configured to restore to its state before being vacuumed after the target to be irradiated on the surface of the sealing bag is removed and the sealing bag is filled with air. a mounting table on which the target to be irradiated is placed; a supporting portion for supporting the mounting table; wherein the system further comprises a positioning assembly provided on the mounting table for positioning the target to be irradiated, wherein the positioning assembly comprises: a shielding body including polymer and radiation shielding material capable of shielding the radiation, wherein the shielding body is made of silicone, radiation shielding material and silicone curing agent, and is in a form of solid particles; and a sealing bag for accommodating the shielding body, a shielding body including polymer and radiation shielding material capable of shielding the radiation, wherein the shielding body is made of silicone, radiation shielding material and silicone curing agent, and is in a form of solid particles; and a sealing bag for accommodating the shielding body, wherein the radiation irradiation system is a neutron capture therapy system, the radiation irradiation device is a neutron capture therapy device including a neutron source for generating a mixed radiation field including neutrons and photons, the shielding material is neutron capture material, the neutron capture material is capable of shielding neutrons and is made of at least one of a boron-containing compound and a lithium-containing compound, wherein the positioning assembly is configured to be recessed with a shape of the target at the position where the target is placed and form a contour corresponding to the target to position the target to be irradiated upon placing the target to be irradiated on the positioning assembly and by vacuuming the sealing bag, and wherein the positioning assembly is configured to restore to its state before being vacuumed after the target to be irradiated on the surface of the sealing bag is removed and the sealing bag is filled with air.