The present disclosure relates to systems and methods for reconstructing a PET image. The systems may execute the methods to acquire PET data of a subject. The PET data may include position information of a plurality of coincident events. The plurality of coincident events may include scattering events and random events. The systems may execute the methods to select a portion of the PET data from the PET data based on the position information. The systems may execute the methods to reconstruct a first preliminary image of the subject based on the selected portion of the PET data, and project the first preliminary image. The systems may execute the methods to may determine, based on the PET data and the projection of the first preliminary image, preliminary correction data relating to the scattering events and the random events.