A mechanical linkage system of Positron Emission Tomography and Computed Tomography/Magnetic Resonance Imaging (CT/MRI), the system comprising: a PET host having a PET detector; a CT/MRI host (21, 24) including one of a CT host and an MRI host disposed opposite to the PET host, wherein the CT/MRI host (21, 24) has an aperture larger than an aperture of the PET detector; a scanning bed located to an end of the CT/MRI host (21, 24); and a moving system connected with the PET host and configured such that the PET detector is operable to be moved into or out of a scanning field of view of the CT/MRI host (21, 24), wherein, the PET host comprises a PET detector fixing plate on which the PET detector is installed, and wherein the moving system comprises a transmission device (22, 23) connected with the PET detector fixing plate. characterized in that the transmission device (22, 23) comprises: a driving motor; two or more rotating screws respectively connected with one of an inner side and an outer side of the PET detector fixing plate; respective rotating wheels installed on the rotating screws, wherein a rotating wheel on one of the rotating screws is a main rotating wheel, and the driving motor is in transmission connection with the main rotating wheel; and a driving belt connecting the rotating screws, wherein the rotating screws are evenly distributed along a same circumference of the PET detector fixing plate, and the driving motor and the main rotating wheel are connected by one of a second driving belt of the main rotating wheel and a gear. a PET host having a PET detector; a CT/MRI host (21, 24) including one of a CT host and an MRI host disposed opposite to the PET host, wherein the CT/MRI host (21, 24) has an aperture larger than an aperture of the PET detector; a scanning bed located to an end of the CT/MRI host (21, 24); and a moving system connected with the PET host and configured such that the PET detector is operable to be moved into or out of a scanning field of view of the CT/MRI host (21, 24), wherein, the PET host comprises a PET detector fixing plate on which the PET detector is installed, and wherein the moving system comprises a transmission device (22, 23) connected with the PET detector fixing plate. characterized in that the transmission device (22, 23) comprises: a driving motor; two or more rotating screws respectively connected with one of an inner side and an outer side of the PET detector fixing plate; respective rotating wheels installed on the rotating screws, wherein a rotating wheel on one of the rotating screws is a main rotating wheel, and the driving motor is in transmission connection with the main rotating wheel; and a driving belt connecting the rotating screws, wherein the rotating screws are evenly distributed along a same circumference of the PET detector fixing plate, and the driving motor and the main rotating wheel are connected by one of a second driving belt of the main rotating wheel and a gear. a driving motor; two or more rotating screws respectively connected with one of an inner side and an outer side of the PET detector fixing plate; respective rotating wheels installed on the rotating screws, wherein a rotating wheel on one of the rotating screws is a main rotating wheel, and the driving motor is in transmission connection with the main rotating wheel; and a driving belt connecting the rotating screws, wherein the rotating screws are evenly distributed along a same circumference of the PET detector fixing plate, and the driving motor and the main rotating wheel are connected by one of a second driving belt of the main rotating wheel and a gear.