The calculation of inter- granule contact force in three-dimensional granular systems is a critical and challenging aspect of granular mechanics research. Taking two elastic rubber balls as research object, in-situ flat pressing of Micro-CT experiment is carried out. Based on the Hertzian contact theory and Tatara large deformation contact theory, the contact model of elastic balls is verified, and the theoretical formula of the contact force of elastic balls based on the experiment is obtained. Taking the three-dimensional granular systems as research object, in-situ probe loading experiment of Micro-CT is carried out to obtain the two-dimensional image sequence of the granules, after a series of digital transformations, the digital body images are emerged, the contact force network of the three-dimensional granular systems under different loading states are obtained by constructing pore network models. The contact force distribution and evolution law of the granular systems are analyzed. The connection between the number of strong contacts and the distribution evolution and the stability of the granular systems is explored. The results show that the two elastic ball contact model conforms to the Hertzian contact theory and Tatara large deformation contact theory, the contact force fitting formula based on experiment can characterize the contact force between two granules reasonably and effectively. The contact force of granules under probe loading shows a net-like distribution with the contact point of the indenter as the starting point, and is transmitted to the lower and the surrounding area step by step. The trend of average contact force is consistent with the number of contacts, obvious phase changes are showed, as the number of contacts increases, the frequency of particle intercompression rises, resulting in greater contact forces between granules, eventually stabilizing at approximately 10.5 N. The number of strong contacts accounts for 45% to 50% of the total number of contacts, and the distribution runs through the whole granular systems, supporting the network structure of the granular systems, larger values are concentrated below the indenter showing a forked distribution. During the loading process, an equilibrium point is established at z=14mm, where the number of strong contacts reaches the peak, the network structure of strong contact force is spread over the whole three-dimensional granular systems, establishing the main skeleton to withstand the external load, as loading continues, the overall values of the strong contact forces increase, and their distribution within the granular system becomes more uniform.
