The present disclosure relates to a PET detector and a PET frame. The PET detector may include a plurality of detector modules and a plurality of installing modules configured to install the plurality of detector modules. The plurality of installing modules may be coupled together to form a detector ring. The PET frame may include a detector stabilizing cylinder configured to stabilize a detector and a fixing support configured to support the detector stabilizing cylinder. The detector stabilizing cylinder may be rotatably fixed on the fixing support.