A mechanical linkage system of Positron Emission Tomography (PET) and Computed Tomography/Magnetic Resonance Imaging (CT/MRI), the system comprising: a PET host having a PET detector ; a CT/MRI host including one of a CT host and an MRI host disposed opposite to the PET host , wherein the CT/MRI host has an aperture larger than an aperture of the PET detector ; a scanning bed located to an end of the CT/MRI host ; 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 , 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 connected with the PET detector fixing plate . characterized in that the transmission device 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 including one of a CT host and an MRI host disposed opposite to the PET host , wherein the CT/MRI host has an aperture larger than an aperture of the PET detector ; a scanning bed located to an end of the CT/MRI host ; 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 , 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 connected with the PET detector fixing plate . characterized in that the transmission device 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. <EOS>