The reinforcement load reduction law for highfilled opencut tunnel is simulated by particle flow code(PFC) software, the microscopic parameters variations of vertical stress on the top of opencut, vertical displacement of particles, contact force, vertical deformation of geogrid and porosity are analyzed. The law of reinforcement load reduction by geogrid is revealed from microscopic aspect, and the influence of intensity and layers of geogrid on unloading effect of top opencut tunnel are further studied. The numerical simulation results indicate that: (1) The vertical deformation of geogrid is consistent with that of particles; the deformation of geogrid depends on the settlement difference between the inner and outer soil column on tunnel roof. (2) Within the maximum deformation range that the grille can withstand, the larger the negative settlement difference between the inner and outer soil column, the more significant the tension membrane effect of the geogrid and the better the load reduction effect. (3) The stiffness and the number of layers of geogrid have a certain impact on its load reduction effect, which should be considered according to height of the filled soil and the nature of the filled soil.