The invention discloses a radiotherapy plan designing simulation and three-dimensional dose distribution testing device. The space positions of points A and B are accurately determined through a locating marking ball and a marking ball fixing plate via the device. In an X-ray film or a CT image, accurate development can be carried out, and dose computing reference points can be determined accurately and conveniently. By changing the placing position of a dome source applicator fixing plate and the length of a locating hole in a uterine cavity source applicator fixing plate, uterine cavity source applicators with the effective lengths being 4 cm, 6 cm and 8cm respectively are arranged, and various typical plan schemes of tumor close-range treating can be subjected to simulation designing. Through dosimetry films arranged in the uterine cavity source applicator fixing plate and at the rest of different positions in an inner cavity of a film fixing plate, parallel plane dose distribution in the main range of whole treating plan radiating at the space distance of 1 mm-2 mm can be obtained, the whole plane dose distribution is compared with dose distribution designed by a plan system, and three-dimensional dose distribution testing and verifying in a space are achieved.