Based on data of satellite, Doppler weather radar, regional automatic observation stations, and conventional observation, combined with NCEP/NCAR 6-h reanalysis data (0.25°×0.25°), the causes of the extraordinary rainstorm in Shandong Province produced by Typhoon RUMBIA (1818) and its aftermath, which affect the region for a long time, are analyzed. The findings are as follows. 1) The extreme precipitation can be divided into three stages, in which the rain areas are affected by the spiral cloud system around the typhoon, the inverted trough, and the cold front of a denatured extratropical cyclone, respectively. The interaction between the weak cold air and the typhoon inverted trough plays an important role in the generation and maintenance of heavy precipitation. 2) During the slow northward movement of Typhoon RUMBIA, the heavy precipitation area contrarotates from the east side of the typhoon to the northern inverted trough of it and gradually moves away from the typhoon center. Furthermore, the intensity of the typhoon is gradually weakened. 3) The cold air interacts with the typhoon inverted trough in the middle troposphere, where the cold and warm advection are strengthened, the frontal zone is formed, and baroclinic instability is intensified. The lifting of warm and wet air near the frontal zone, combined with the convergence and upward movement of the inverted trough at the lower layer, generates the extraordinary rainstorm near the inverted trough. 4) In this process, the stable low-level jet stream continuously carries water vapor from the East China Sea to the typhoon inverted trough and the transfer of water vapor is concentrated below 800 hPa. The region where the intensity of water vapor flux convergence at 850 hPa is greater than 8×10-6 g·cm-2·hPa-1·s-1 corresponds well with the shape and location of the rainstorm area. 5) The weak cold air in the middle troposphere and warm and wet air at the lower layer promote the development and maintenance of instable convective stratification. Under the guidance of middle and upper airflow and the impact of topography, mesoscale convective system is constantly triggered by the forced uplift of the strong wind belt at the lower layer in the windward slope of central mountainous areas in Shandong Province, producing train-effect, which is also an important factor for the occurrence of local extraordinary rainstorm in this process.