Based on intensive observations of automatic weather stations, Jinan dual-polarization radar data and ERA5 reanalysis data, this study investigates the diurnal variation characteristics and mechanism of a heavy precipitation event over the mountainous areas of central Shandong on 21 July 2025 under the influence of a weak convergence line on the periphery of the subtropical high. It is found that the boundary layer inertial oscillation is the primary cause of the diurnal variations of the precipitation, playing a key role in the nocturnal precipitation enhancement. The results are outlined below. (1) This precipitation event exhibits significant diurnal variations, with its peak occurring at night, while the precipitation intensity and coverage weaken markedly during the day. Influenced by the combined effects of the precipitation system movement and the convergence line strength evolution, a phase difference exists in the timing of the precipitation peaks between the southern and northern sides of the mountains. (2) The boundary layer inertial oscillation causes a pronounced clockwise rotation of the low-level wind field over a 24-h period, leading to periodic evolutions in the intensity and location of the low-level wind speed convergence zone on the periphery of the subtropical high. Such evolutions cooperate with the local lifting effects of the mountainous terrain of central Shandong, resulting in periodic variations in the rainband location and its intensity, thereby determining the timing of the diurnal precipitation peaks. Around dawn, a rapid increase in the ageostrophic southerly wind component near the bell-mouthed terrain on the southern side of the mountains enhances the lifting effects of the mountainous terrain, favorable for triggering extreme hourly precipitation there. (3) The low-level circulation affects the local precipitation intensity by modulating the low-level moisture content. Around early morning, the strengthening of the southerly wind on the southern-northwestern side of the mountains enhances the low-level convergence on the northern side of the mountains. However, it also rapidly advects the air particles—which have already depleted significant amount of water vapor over the heavy precipitation zone on the southern side—to the northern side. This reduces the local moisture content, thus preventing significant precipitation on the northern side despite the favorable dynamic lifting.