Abstract: The diurnal variation of mesoscale convective systems (MCS) and associated precipitation in the Beijing-Tianjin-Hebei area during the summers of 2008–2018 are analyzed using the MCS database of East China, the hourly merged precipitation product of China, and the ERA-5 reanalysis data, with a particular focus on its mountain-departing propagation mechanisms. The study reveals that the diurnal variation of MCS intensity in Beijing-Tianjin-Hebei region is significant, which is characterized by a unimodal peak around 19: 00 BT (Beijing Time), with the total precipitation reaching its maximum around 20: 00 BT. Spatially, MCSs exhibit 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 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 the baroclinic term primarily influence the process through clockwise rotation of the ageostrophic U-component, which steers the gradual propagation of ageostrophic convergence from mountains to plains. The inertial advection term facilitates the eastward propagation of MCS over the plains, which is resulted from the combined effects of eastward-propagating ageostrophic U-component and northward-enhanced V-component. 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 foothill of the Taihang mountains between 22: 00 and 04: 00 BT and promoting nocturnal MCS precipitation. These results highlight the important influences of boundary layer processes on the movement and development of MCSs.