The simulation of the extremely severe rainfall that occurred in Zhengzhou City, Henan Province, on 20 July 2021, is carried out to figure out the effects of topographic height and relief on water vapour transport and vertical conditions using the numerical model. The result shows that the CMA-MESO (GRAPES-MESO 3km) model’s simulation was satisfied with the actual situation for this process. However, the simulated precipitation centre was slightly to the west. The terrain had a significant impact on precipitation. The precipitation centre weakened when the terrain height decreased, and its position leaned toward the north. The precipitation centre strengthened when the terrain height increased, and its position leaned toward the south. The main influence mechanisms are as follows: (1) The upward effect of the Taihang Mountain and the mountain on the west side of Zhengzhou City resulted in a strong vertical ascending movement in the west of Zhengzhou City, and there was a deep convective system. The blocking effect of Funiu Mountain terrain had a significant effect on the north-south position of the vertical movement centre. Under the influence of the two typhoons, the warm and wet air coming from the southeast was formed, and the blocking effect of the southern part of Taihang Mountain caused part of the air in the north branch to deflect to the west and produced a southward component. That conjoined with the southerly airflow over Funiu Mountain and the southern branch of the southeast airflow, transported water vapour to the northwest under the blocking mountains such as Taihang Mountain and Songshan Mountain so that large-scale water vapour convergence was maintained over Zhengzhou City, and extremely heavy rain was generated. (2) With the increase of local height, part of the airflow from the north branch of the southeast airflow was blocked and turned into the northeast airflow, the intensity of the convergence zone of the airflow increased, the vertical velocity centre shifted to the south, and the water vapour content over Zhengzhou increased obviously. The southern branch of the southeast airflow also produced a strong updraft in the southwest of Zhengzhou under the effect of Funiu Mountain and formed two heavy precipitation centres near Zhengzhou, corresponding to Funiu Mountain and Songshan Mountain in the southwest and Taihang Mountain in the northwest, respectively. Under the uniform terrain condition, without the lifting and blocking effect of Taihang Mountain and Songshan Mountain, the water vapour accumulation over Zhengzhou decreased, the vertical movement centre moved northward, and the simulated precipitation centre was weaker than the real terrain condition, and the location was northward.