The invention relates to an accelerator and an MRI(magnetic resonance imaging)-guided X-ray radiotherapy machine. The accelerator is provided with a magnetron, the magnetron comprises a tube core and a magnet, the magnet employs a master magnet of an MRI system, the tube core is arranged in a main magnetic field, generated by the main magnet, of the MRI system, the axial direction of the tube core is along the direction of the magnetic line of the main magnetic field; and the MRI-guided radiotherapy machine is composed of a radiotherapy machine and an online MRI device, the radiotherapy machine is provided with a linear accelerator and/or a cyclotron for acceleration of charged particles, the linear accelerator and the cyclotron are provided with a magnetron, the online MRI device is provided with a main magnet for forming an MRI main magnetic field, the tube core of the magnetron is arranged in the MRI main magnetic field, the MRI main magnetic field is taken as a constant magnetic field of the magnetron, and the linear accelerator is disposed outside the main magnet. The accelerator provided by the invention is smaller and more compact and generates higher-energy electron beams; and the radiotherapy machine is more compact and can rapidly and accurately deliver the beams to specific irradiation positions.