Abstract: The diurnal variation of mesoscale convective systems (MCS) and its precipitation in the Beijing-Tianjin-Hebei area during the summer of 2008–2018 were analyzed using the MCS database of Eastern China, the hourly merged precipitation product of China, and the ERA-5 reanalysis data, with a particular focus on exploring its mountain-departing propagation mechanisms. The study found that the diurnal variation of MCS intensity in Beijing-Tianjin-Hebei region are significant, with MCS showing a unimodal structure around 1900 BT, and the total precipitation reaching its maximum around 2000 BT. Spatially, MCS exhibits a mountain-departing propagation pattern that is nearly perpendicular to the orientation of the mountains. Further diagnostic analysis of the ageostrophic wind equation reveals that the mountain-departing propagation of MCSs results from the combined effects of the local tendency term, inertial advection term, and baroclinic term, with the convergence contributions from the local tendency term and inertial advection term being relatively more significant than that of the baroclinic term. Both the local tendency term and baroclinic term primarily influence the process through clockwise rotation of the ageostrophic U-component, which guides the gradual propagation of ageostrophic convergence from the mountains to the plains. The inertial advection term facilitates the eastward propagation of MCSs over the plains, resulting from the combined effects of eastward-propagating ageostrophic U-components and northward-enhanced V-components. The boundary layer friction term mainly reflects the dynamic lifting effect of terrain on MCSs, with the strongest convergence induced by boundary layer friction occurring at the foothills of the Taihang Mountains between 2200 and 0400 BT, favoring the enhancement of nocturnal MCS precipitation. These results highlight the important influence of boundary layer processes on the movement and development of MCSs.