Based on the conventional observations data and NCEP/NCAR reanalysis data, the numerical simulation and comparative analysis of the topographic effects on two severe flood rainstorms which occurred on 21 August 2016 and 22 July 2018 in Helan Mountain were carried out by using the weather research and forecasting (WRF) model. The results are as follows: (1) The atmospheric circulation situation during the two heavy rainstorm was basically similar. The eastern part of Northwest China was under the control of air with high temperature, high humidity and high energy. There was subtropical high blocking in the east and cold air moving eastward in the west side of Ningxia. Further more, the lower layer was controlled by southerly and easterly airflow. (2) Under the conditions of high humidity, high energy and extremely unstable atmospheric stratification, when a low-level jet with a wind speed more than 12 m·s-1 appeared in northern Ningxia, and the angle between the wind direction and the windward slope of the eastern foot of Helan Mountain reached 60° -90°, the strong vertical motion was triggered by the terrain blocking, which made the warm and humid unstable energy accumulate and release rapidly. The duration of the rainstorm depended on the duration of the low-level jet. (3) The joint effect of the trumpet-shape topography of Suyukou and the main peak of Helan Mountain in the north side of Suyukou on the low-level jet led to the massive accumulation and strong uplift of water vapor, which determined the location of the rainstorm center.