Abstract: Scattered coincidences could be harmful to the quality of the image reconstruction.The lower level discriminator can decrease the proportion of scattered coincidence counts.This paper is to validate the effect of the lower level discriminator on the coincidence counts.GATE(geant4 application for tomographic emission) was used to find the best value of the lower level discriminator by modeling the Eplus-166 PET scanner developed by IHEP(Institute of High Energy Physics) in China.It is found that the value of the threshold should be set properly due to the size of the creature to be scanned.Compared with the experiment data,to simulate PET with GATE is practicable.