1. A positron emission tomography (PET) imaging method; wherein used equipment comprises a computer imaging system, a device carrying a detected object, and a detector system disposed around the detected object for collecting gamma photons emitted in a positron annihilation event; wherein a multi-pinhole plate is disposed between the detected object and a detector, and the detected object is traced by a radioactive isotope of β+ decay; the imaging method comprising the following steps: 1) collecting, by the detector, gamma photon projections from a plurality of angles, a projection image is denoted by p(x, y), where x and y denote a coordinate of two dimensions of the detector; and 2) reconstructing a three-dimensional image based on all the projection images.