Abstract ： Research purposes ： The ballast is the key element of ballast track，and its accumulative deformation will directly cause the damage and settlement of track. With the improvement of train speed，the effect of ballast deformation on running safety，comfort and workload of main tenancies ballast track is growing. Therefore，it is of great significance to carry out the research on the mechanism of high - speed railway ballast deformation. In recent years， in order to reduce the settlement deformation of railway ballast，some scholars began to apply geotechnical grid to the reinforcement of railway ballast，but it is not enough mature and perfect in general. In order to explore the deformation mechanism of baiasts and study the effect of georgic reinforcing railway ballast，the 3-D particle discrete element model of georgic reinforcing railway ballast was established，and numerical tests of the model under cyclic loading were carried out. The influences of geogrid with different locations，different stiffness and different grid size on the reinforcing effect were researched respectively and compared with the related test results. Research conclusions:(1) The effect of geogrid reinforcing railway ballast is obvious，which can effectively reduce the settlement deformation of railway ballast. (2) The effect of reinforcement varies with the location of geogrid. When the distance between geogrid and the bottom of model box is 100 mm，the effect is best. (3) With the increase of stiffness of geogrid, the settlement deformation of ballast will decrease. But when the stiffness increases to a certain extent，the effect is not obvious with further increase of stiffness. (4) The influence of the grid size on the reinforcing effect is obvious. However， the effect is not monotonous， which means that there exists a grid size to achieve the optimal effectiveness. (4) The research result is helpful to understand deeply the settlement mechanism of railway baiast and the application of geogrid reinforcing railway ballast