Abstract: Under the influence of No. 18 Typhoon “Rumbia”, a rarely-seen extreme rainstorm occurred in the eastern area of Henan Province during 17-19 August 2018. The movement speed of the typhoon low pressure slowed down significantly after entering Henan in the early morning of the 18th and stayed there for 40 hours, which resulted in strong wind and heavy rain in Henan. The daily precipitation of 8 national meteorological stations exceeded the historical extreme value since the establishment of the station. In order to fully understand the long-term retention of typhoon low pressure and the occurrence and development mechanism of extreme rainstorms in eastern Henan, the paper conducts a comprehensive and detailed diagnostic analysis of the typhoon and the rainstorm process during 17-19 August 2018 from the circulation background, atmospheric stability, dynamics, thermodynamics and water vapor by using conventional sounding and ground observation data and NCEP 1°×1° reanalysis data.The results show that the center of the heavy precipitation is located in the spiral cloud belt about 250 km away from the low pressure and the strongly developed ramp-up airflow is the fundamental cause of the heavy precipitation. Prior to the extreme rainfall process, the atmosphere already has the energy and instability for the occurrence and development of strong convection.The inclined development mechanism has been maintained for a long time, accompanied by sufficient water vapor supply and strong convergence and upward movement, the continuous generation of convective cells, the merger and development of strong thunderstorm cells, and the stable and infrequent movement of strong echo wavelengths. These are the direct reasons for the occurrence of the heavy precipitation. The weakening of the high-level guided airflow and the blocking of the middle and low-level high pressure belts make the typhoon low pressure linger in Henan Province for a long time. Comparing the standard deviation multiples of relevant environmental parameters with the average and maximum values over the years when heavy rainfall events occurred at Shangqiu Station, it is found that the standard deviation multiples of each environmental parameter in this process of extreme rainfall are significantly higher than the average value over the years and some of them have reached the historical maximum value, exhibiting certain level of extremity.