Abstract: Modern radiotherapy requires new approaches to dosimetry.The dosimeters must be able to assess the dose not only in a point but also in complex three-dimensional objects.Recently 3D Gel dosimetry using a tissue-equivalence phantom to record dose stereo-distribution then to acquire 3D dose data with MRI and optical evaluation techniques is a highly promising tool to address the emerging needs of radiotherapy dosimetry.An optical CT system used to scanning 3D dose distribution in a polymer gel has been designed.The optical CT system has a cone light beam(580nm) scanner,a CCD camera to acquire the project data and a PC for scan control and for mapping of 3D dose distributions.Scan time and reconstruct time,resolution and dose respond are practically satisfaction.The polymer gel dosimeter is a MAGIC phantom.The irradiation of gel dosimeter was performed using a 6 MV accelerator X-ray with a field size of 3 cm x 3 cm.The dose information is stable.The dose distribution map acquired is satisfaction.