Based on summer 500hPa geo-potential high fields,mid-latitude blocking high indexes of three areas (Okhotsk Sea,Baikal Lake,Ural Mountains) in Asia are calculated in this paper and the relation between blocking high indexes and summer precipitation in North China is investigated by means of correlation and composite analysis.Results show that the blocking high indexes of the three areas all have distinct inter-decadal variations,and are close related with summer precipitation in North China on inter-decadal time scales,with correlation coefficents being -0.4622,-0.6763 and -0.6713,respectively.When the Asian mid-latitude blocking high occurs frequently,the mid-latitude westerly turns into two branches,forming polar and sub-tropical fronts,and the East Asian summer monsoon is weaker than normal,the sub-tropical high lies south of normal position,and the summer rain-belt lies south of normal,resulting in summer drought in North China.It is vice versa if the Asian mid-latitude blocking high occurs infrequently.