Abstract: The pullout tests on geogrids have been regarded as the most direct way to investigate the geogrid-soil interaction. In the pullout tests on geogrids, either a rigid or a flexible top boundary is commonly used for applying the vertical loads. In order to investigates the influence of rigid and flexible top boundaries on pullout test results, discrete element modeling is carried out to deeply analyze the mechanical response of geogrids and soils under rigid and flexible top boundary conditions from the mesoscopic scale. The results show that the rigid and flexible top boundaries have almost no effects on the pullout test results when the pullout displacement is small. With the increasing pullout displacement, the influences of the top boundaries on the pullout test results become obvious. The maximum pullout resistance under the rigid top boundary conditions is slightly larger than that under flexible ones. Moreover, the influences of the top boundaries on the pullout test results can be visualized by the tensile force distribution of geogrids, the contact force distribution of soil particles in the specimens, the rotations of soil particles and the vertical displacement distribution of top loading particles. The investigation results in this study may provide scientific references for regulating the pullout test methods for geogrids.