The invention relates to the technical field of ink formulation, specifically to a preparation method of ink for a special radiotherapy stylus. The ink for the special radiotherapy special stylus is prepared from the following components in parts by mass: 5 to 8 parts of pure essential oil, 5 to 10 parts of lemon concentrate, 1 to 2 parts of glory red henna, 0.5 to 1 part of vitamin, 3 to 5 partsof gentian violet and 80 to 90 parts of genipa americana juice. The ink for the special radiotherapy stylus, prepared by the invention, has higher coloring capability and only needs one time to mark clearly; meanwhile, the ink has durable coloring effect and does not fade in a short period of time so as to avoid the influence on the conduct of radiotherapy; in addition, raw materials used in the invention are all natural plant extraction substances, so that the ink is a safe, environment-friendly and non-toxic product.