The possible reason for the aridification of the representative summer month (July) in the east of NW China in the last twenty years is investigated by a method combined with the EOF,probability density function and the similarity of two fields.It is for the first time pointed out that there exists two weather regimes in July East Asian atmospheric general circulation,namely the A regime and B regime.The A regime is associated with the drought circulation pattern for the east of northwest China,and the B regime with the comparatively wet pattern.The possible reason for the aridification of summer in the east region of NW China in recent 20 years is that the frequency of positive similarity with the A regime (drought pattern) persistently increases and the frequency of positive similarity with the B regime (comparatively wet pattern) persistently decreases.