Abstract: As a necessary composition of medical CT image, in some cases, CT number has been an aid when doctors diagnose diseases, such as measurement of bone density, image segmentation and recognition. This paper described a CT number conversion method of cone beam CT. On the base of Sedentex CT phantom, standard CT numbers and reconstructed attenuation coefficients of five analyzed materials were obtained respectively. The conversion equation between CT number and reconstructed attenuation coefficient was established by linear fitting. After CT number transformation, the numerical changes of contrast to noise ratio and uniform deviation were analyzed in the end.