By employing the NCEP/NCAR reanalysis data, the characteristics of the dry cold air activity, the dry cold air intrusion and evolution, as well as its impacts o n the torrential rain during the 2008 Meiyu period are diagnosed．It is shown th at there exists a close relationship between the evolution of the circulation ov er the midhigh latitudes and the torrential rain process, and the establishmen t and maintenance of the blocking anticyclone (pressure ridge) over Northern A sia lead to the southward movement of the westerly frontal zone and the dry co ld air. The abnormal eastward movement and the enhancement of the polar cyclone transfer the dry cold air to the Changjiang River and the Huaihe River Basins, t hus enhance the torrential rain over that area. It is also revealed that the nor therly activities are in coherence with the torrential rain process, the dry cold air comes from the midhigh latitudes and the midhigh level. The Cha ngjiang River and the Huaihe River Basins locate in the low level air convergenc e area. The north wind plays an important role in the enhancement of the converg ence in the lower level and the divergence in the upper leve1.The dry cold air i ntrusion contributes to the formation and maintenance of the dry level. The dry cold air intrusion is one of the important dynamical and thermal causes for the formation and maintenance of moist front during the Meiyu period. At last, the k eys of midrange forecast are summarized on the basis of these results.