An infrequent big hail event occurring on 29th, Aug. 1969 is analyzed. A trough from Mongolia at 500hPa plays a triggering role. And the upper level c old air accompanying with the trough superposes the low level warm air, which le ads to some instability. The calculation of CAPE and LI indicates that the hails fall in the instable region and Beijing is in the center of this zone with unus ual great CAPE and LI. In addition, the strong wind shear in the low troposphere and the value of CAPE in the layer between -10℃ and -30℃ are both favorable to the big hail event. Finally, the suitable freezing level height and the ratio of cold and warm cloud thickness are also favorable to the hail shooting.