The invention relates to an adjustable neutron tube fixing device and method. The distance between a movable disc and a fixed disc is adjusted by turning a small lead screw, the mutual distances among three clamping claws installed on the fixed disc and the movable disc respectively are simultaneously changed by turning a shifting rod, a neutron tube extends into a space formed by the six claws, the shifting rod is loosened, and the clamping claws fix the neutron tube under the effects of a torsion spring. After the neutron tube is fixed well, the relative position of the neutron tube and an instrument device needing the neutron tube is adjusted by turning the lead screw, so that the neutron tube is located at the best usage position.