Methods, devices and apparatus for reconstructing a PET image are provided. According to an example of the method, a PET initial image may be reconstructed from PET data obtained by scanning on a target object with a PET device, and an MRI image may be reconstructed from MRI data obtained by scanning on the target object with a MRI device, a fusion image retaining only a boundary of the target object is generated based on the PET initial image and the MRI image, and a PET reconstructed image is obtained by combining the PET initial image with the fusion image, where the definition of the boundary of the target object in the PET reconstructed image is higher than that in the PET initial image.