A mechanical linkage system of PET and CT/MRI and a linkage scanning method thereof are disclosed. According to an example of the disclosure, the mechanical linkage system includes a PET host, a CT/MRI host, a scanning bed, and a moving system. The PET host includes a PET detector installed in a PET detector fixing plate and configured to form a positron emission computed tomography image. The CT/MRI host has a larger aperture than the PET detector. A front end of the PET host and a front end of the CT/MRI host are oppositely disposed. The scanning bed is located to an end of the CT/MRI host. The moving system is connected with at least one of the PET host or the CT/MRI host and configured to move the PET detector into or out of a scanning field of view of the CT/MRI host.