1. A system for reusing triple event data of positron emission tomography, the system comprising: a positron emission tomography scan device; an input unit, used to receive a plurality of data generated from the positron emission tomography scan device, wherein the data comprising different events with photons detected; a determining unit, connected with the input unit, dropping out the data corresponding to single event, picking out the data corresponding to double event, then picking out the data corresponding to triple event from the data, and calculating three parameters of time difference of three photons detected by the positron emission tomography scan device, three lengths of line of response (LOR) and total activity of the three LOR through the object in triple event to obtain three first probability values by a formula, determining whether one of the largest first probability value is greater than twice of the sum of the other two first probability values, if yes, picking out the data with the largest first probability value, if not, dropping out the data, and analyzing the data which have not been dropped out to generate an analysis result; and an output unit, connected with the determining unit to output the analysis result.