The invention involves an imaging device used in rotary focused gamma-ray radiotherapy. The invention includes a frame, a rotary ring positioned at the frame, and a source carrier and a collimator carrier that can rotate around a rotary axis respectively. The source carrier is equipped inside with multiple radiation sources. The collimator carrier is equipped inside with a corresponding beam channel to the radiation sources, which focus at a shared focus of the rotary axis through the beam channel. The imaging device of the radiotherapy system also includes a diagnostic imaging system, consists of diagnostic radiation sources, a detector and a data processing system. By means of diagnostic imaging system of the invention, automatic beam positioning, real-time monitoring of treatment target motion, real-time monitoring and verification of radiation dose can be done during treatment, ensuring more accurate and improved radiotherapy effect.