1. A method of online energy coincidence for an all-digital PET system, wherein the method comprises: S1: collecting information of scintillation pulses generated by incident γ photons, forming a data frame of a single event and transmitting the data frame to a host computer, by a detector module, wherein: the detector module comprises a plurality of crystal strips, each of the plurality of crystal strips is identified by a number of the crystal strip, wherein the information of scintillation pulses comprises time, energy and position information, and wherein the single event refers to hitting of an incident γ photon to one of the plurality of the crystal strips; S2: performing two-dimensional position distribution statistics on the incident γ photons to acquire X, Y two-dimensional coordinates of each of the incident γ photons which form a two-dimensional position spectrum of the incident γ photons, performing segmentation of the position spectrum to map X, Y two-dimensional coordinates to the numbers of the plurality of crystal strips, and forming a crystal segmentation table, by the host computer; S3: after the segmentation, calculating the energy information included in the data frame of the single event to acquire an energy distribution spectrum of the scintillation pulses contained in each of the crystal strips, obtaining an energy value, as an energy correction value of a corresponding crystal strip, that actually corresponds to a 511 keV energy peak for each of the energy distribution spectrums by searching for a point at which count of each of the energy spectrums is the highest, to form an energy correction table, by the host computer; S4: transmitting the crystal segmentation table and the energy correction table from the host computer to a storage unit of the detector module; S5: collecting information of online energy correction by the detector module according to the crystal segmentation table and the energy correction table; S6: when a single event occurs, searching the crystal segmentation table to obtain the number of a corresponding crystal strip which maps to the single event in accordance with a two-dimensional coordinate of the single event, and searching the energy correction table to obtain an energy correction value in accordance with the number of the corresponding crystal strip, by the detector module, wherein the crystal segmentation table and the energy correction table are in a format of lookup table; and S7: calculating a true energy value of the single event in accordance with a searched energy correction value, reserving the single event if the true energy value of the single event is within an energy window, filtering out the single event if the true energy value of the single event is not within the energy window, and transmitting data of the reserved single event to the host computer, by the detector module.