The geogrid reinforced foundation is a foundation treatment method to improve the bearing capacity of the foundation by laying appropriate geogrids at a certain depth under the footing. Based on model tests, a 3D particle flow code (PFC3D) was applied to simulate the geogrid reinforced foundation under strip footing. The bearing capacity and the reinforcement mechanism of the geogrid reinforced foundation were investigated by analyzing stress distribution, sand particle displacement, sand particle contact force, geogrid deformation and geogrid-sand interaction, et al. The results demonstrated that a more uniform loading was applied on the foundation by using the geogrid reinforcement, and the effective reinforcement length was approximately 1.3~1.5 times of the footing width. The primary reinforcement involved surrounding soil materials to interlock across the geogrid plane. This interlocking enhanced passive resistance, rendering the geogrid-reinforced foundation more integral. Consequently, stress redistribution occurred, leading to an improvement in bearing capacity. In addition, the lateral restraint of the transverse ribs was the main reason for inducing a tensile force in the longitudinal ribs. The deformation of geogrid had a reverse bending point under the applied loading. An upward and downward force was applied to the sand particles at the two sides of the reverse bending point, respectively. The tension membrane mechanism of the geogrid decreased the uneven settlement in the overall geogrid reinforced foundation.