Diagnostic analysis and numeric simulation were made on a heavy rain process in the southeast part of Gansu during 1 － 2 July
2005 using NCEP data of 1° × 1° with 6 h intervals and WRFV3． 1 meso － scale model． Based on the data of model output，the relation
between the helicity of this process and the rainfall intensity was analyzed． The results show that the short － wave trough from Plateau
and the moisture from South China Sea and Bengal were important causes of the heavy rain process． WRF model had the ability of
simulating this heavy rain’s precipitation; The spatial distribution of the helicity had certain indication about prediction of heavy rain in
eastern part of northwest China． The shape of maximum value center of positive helicity correlated with the shapes of short － wave trough
from Plateau and shear line on 700 hPa; The maximum value center of positive helicity which occurred under 400 hPa may be a great
cause for heavy rain in northwest part of China．