Based on high-resolution multi-source observations, this study examines the characteristics and causes of the extreme rainstorm event that occurred on 21–22 July 2025 over the complex terrain of the Central Shandong Mountainous Region. The "7·22" rainstorm was a long-duration, high-intensity precipitation event driven by the combined influence of the subtropical high, a remote typhoon, and a low-level shear line. Daily precipitation exceeding 100 mm was concentrated along the mountains, with a core area of more than 250 mm located inside a horn-shaped topographic zone on the southern flank of the range. The complex underlying surface and the concentrated nature of the heavy rainfall contributed to the severe flooding in the region. Analysis indicated that abundant moisture supply was a critical condition for this extreme rainstorm event. A moisture transport channel formed between the subtropical high and Typhoon "Wipha" resulted in a moderately stronger positive anomaly in the total atmospheric precipitable water over the central Shandong mountainous area compared to the climatic norm. Areas of high pseudo-equivalent potential temperature and moisture flux divergence were distributed along the mountain range. The mountain-crossing airflow was forced to uplift by the terrain, triggering local mesoscale convection. Systematic precipitation and local convective precipitation overlapped in the near-mountain areas. Radar data revealed a strengthening low-level jet on the mountains’ southern side, where convective cells repeatedly formed and merged into the main rainband. Topographic blocking sustained a mesoscale convective system (MCS), while successive mesoscale vortices moving through the horn-shaped area enhanced low-level convergence and uplift. Overlap of vortex tracks with heavy rainfall echoes produced a train effect, yielding a 3-hour accumulation of 268.1?mm at Shiyunshan Station in Laiwu. This study advances the understanding of complex terrain effects on extreme rainstorms and offers valuable insights for future forecasting and risk management.