Hydrogen storage remains one of the main challenges in the implementation of a hydrogen-based energy economy. Among various porous materials, hydrogen storage in covalent-organic frameworks (COFs) has attracted the most significant attention since they were first synthesized due to good stability, large surface area, porosity and extremely low density. Although COFs exhibit promising hydrogen storage properties at very low temperatures, their hydrogen storage capacity is not satisfactory at room temperature, which is too low to meet the uptake target set by US-DOE, thereby being unable to have practical applications. Remarkably, hydrogen spillover has been experimentally demonstrated as an effective approach to improving the hydrogen storage capacity on porous materials at ambient temperature. In some of the most promising results the metal-organic frameworks (MOFs) and COFs have been used as substrates. However, the structures of many COFs materials are quite complex and the experimental condition is difficult to control. Furthermore, the sample preparations for these hydrogen spillover experiments are also very difficult. Therefore, only COF-1 is used in experimental study of hydrogen spillover. Although some theoretical work has contributed to understanding the hydrogen spillover mechanism of COFs, many basic problems about hydrogen spillover have not been solved, which hinders its practical application to a large extent. Based on the above reasons, the hydrogen spillover mechanism of Pd doped COF-108 is studied by using density functional theory (DFT) method, which mainly includes the various deposited configurations of Pd4 cluster on COF-108, the adsorption and dissociation of H2 on Pd4 cluster of Pd4@COF-108, the migration of H atom from Pd4 cluster toward the COF-108 and the diffusion of H atom on COF-108 surface. The results show as follows. 1) The larger the number of Pd atoms interacting with HHTP or TBPM cluster, the greater the binding energy of Pd4 deposited on them is. Deposited configuration orientation has little effect on binding energy. The binding energies of all deposition configurations for TBPM cluster are larger than those for HHTP cluster, so Pd4 cluster prefers to deposit on TBPM cluster with face-contact configuration. (2) H2 molecules spontaneously dissociated into Pd4 cluster, i.e., a barrierless H2 dissociation process takes place, which meets the first condition required by hydrogen spillover. 3) Only H atom located at the bridge site may migrate to the substrate surface, and the migration process is an endothermic reaction and less stable, which indicates that H atoms will further diffuse on the substrate surface. Although H atoms located at the top site may not migrate directly to the substrate surface, it will automatically migrate to the bridge site after the H atom on the bridge site has migrated to the substrate surface, so the migration process may proceed continuously. (4) The introduction of transition metal Pd can greatly reduce the diffusion energy barrier of H atoms on substrate surface, which makes it easier for H atoms to diffuse on substrate. These results may help us understand the microscopic mechanism of hydrogen spillover influencing the properties of hydrogen storage on COFs and provide useful guidance for targeted preparing the COFs materials with excellent hydrogen storage properties experimentally.