Abstract: In industry X-ray CT detection system,when the object is tested,the rack which is carrying the object would continue to rotate.The mechanical radical runout would follow it during the rotating process.The photon's number would be influenced by the radical runout,and then the projection data would be changed.In the end,the reconstructed images would be affected.This paper has mainly analyzed the reasons why the radical runout happened and the influence of the event to X-ray photons which have been received by the detectors.Based on the above analysis,a new CT projection formula about radical runout had been proposed and then the projection had been simulated.