The invention belongs to the technical field of radioactive source preparation and discloses a method used for preparing a large-area radioactive source. The method comprises the following steps: placing a plating solution on a plating plate with a protection frame, wherein the plating plate is a cathode, and the mirror side of the plating plate is used as an electroplating surface; under the synergistic action of a lifting platform car, a leveling platform, a plating pen holder and a gauge block, adjusting parallelism between the plating plate and a plating pen, adjusting the distance between the plating plate and the lower end of the plating pen, and keeping the distance at 6mm; adjusting the initial position of the plating pen, setting plating pen movement speed, movement distance and other related parameters required on two-dimensional translation device control software, utilizing a computer and the two-dimensional translation device control software to carry out remote control on the stroke of the plating pen, and ensuring that the plating pen is not in contact with the plating solution protection frame in the whole stroke; and setting power supply voltage values and current values according to the types of plated nuclide, and electroplating the plating plate so as to obtain a large-area radioactive source. The method has the advantages of being great in evenness, high in electro-deposition efficiency, simple to operate and low in cost and also has the advantage that the area of the active zone can be as large as 1000cm<2>.