Conventional meteorological data, Yinchuan CD Doppler radar and ECMWF, T639, WRF, NCEP/NCAR data are used to analyze the causes, predictability and significant forecast error margin of the rare severe rainstorm along the eastern Helan Mountain on 21 August 2016, which was missed by Ningxia Meteorological Observatory. The results show that the ECMWF, T639, WRF models predicted that there would be precipitation below 15 mm in the midnorth of Ningxia, but the magnitude was significantly smaller. Among the three models, the effect of ECMWF forecast was obviously superior to the other two. In the mid and late August 2016, the subtropical high was stronger continuously, located by north and getting to the strongest on the 21st. Under the control of the 588 dagpm line of the subtropical high, when the 592 dagpm line rapidly retreated eastward in southern Ningxia, the interaction of boundary layer shear line, lowlevel convergence and jet as well as the terrain of Helan Mountain triggered this severe rainstorm. Lack of experience, the local forecasters failed to capture the evolution of the circulation. The southeast and southerly lowlevel jet were formed when the stream in lower and middle layers were westbound, which was originated from the East China Sea, and established a warm convergence line in the eastern foot of Helan Mountain. The terrain friction convergence, the uplift of the wind and the movement of the mountain blocked the flow of Helan Mountain, and extended the rainfall time, contributing to the intensification of the rainstorm. Unfortunately, forecasters neglected the impact of Helan Mountain terrain on the rainfall.They didn’t grasp the forecast indicators, such as K index≥38℃, LI≤-3, Q≥12 kg·kg-1.By the analysis, the predictability of the severe rainstorm on 21 August was put forward.