Based on high-resolution multi-source observations, this study examines the characteristics and causes of the extreme rainstorm event that occurred from 21 to 22 July 2025 under the effect of the complex terrain over the mountainous areas of central Shandong. The results are outlined below. (1) It is a long-duration and high-intensity precipitation event driven by the combined impact of the subtropical high, a remote typhoon and a low-level shear line. The areas with daily precipitation exceeding 100 mm are concentrated along the mountains, with a core area of precipitation more than 250 mm located inside a bell mouth-shaped area on the southern flank of the mountains. The complex underlying surface and the concentrated nature of heavy rainfall contribute to the disastrous rainstorm over the mountainous areas of central Shandong. (2) The abundant water vapor supply is a critical condition for this extreme rainstorm event. A water vapor transport channel forms between the subtropical high and Typhoon Wipha, resulting in a moderately stronger positive anomaly in the total atmospheric precipitable water over the mountainous areas of central Shandong compared to the climatological normal. (3) The distribution of pseudo-equivalent potential temperature and water vapor flux divergence shows topographic correlation. The mountain-crossing airflow is forced to ascend by the terrain, triggering local mesoscale convection. Systematic precipitation overlaps with local convective precipitation near the mountains. (4) The radar data reveals a strengthening low-level jet on the mountains' southern side, where convective cells repeatedly form and merge into the main rainband. Topographic blocking sustains a mesoscale convective system (MCS), while successive mesoscale vortices moving through the bell mouth-shaped area enhance the low-level convergence and ascending motion. The overlap of vortex tracks with heavy rainfall echoes produces a train effect, yielding a 3-h accumulated precipitation of up to 268.1 mm at Shiyunshan station in Laiwu. This study advances the understanding of the effect of the complex terrain on extreme rainstorms over the mountainous areas of central Shandong and offers valuable insights for future forecasting and risk management in relevant areas.