Abstract: Using the NCAR/NCEP daily reanalysis data from 1 Dec 2004 to 28 Feb 2005, the isentropic potential vorticity (IPV) analysis of a strong countrywide cold wave from 22 Dec 2004 to 1 Jan 2005 was performed. It is found that the strong cold air of the cold wave originated from the lower stratosphere and upper-troposphere of the high latitude in the Eurasian continent and the Arctic area. Before the outbreak of the cold wave, the strong cold air of high PV propagated down to the south of Lake Baikal, and was cut off by the low PV air of low latitude origin, forming a dipole-type circulation pattern with the low PV center (blocking high) in the northern part of the Eurasian continent and the high PV one (low vortex) in the southern part. Along with the decaying of the low PV center, the high PV center (strong cold air) moved towards the southeast along the north flank of the Tibetan Plateau. When arrived East China, the air column of high PV rapidly stretched downwards, leading to increase in its cyclonic vorticity, which made the East Asian Trough deepened rapidly, and finally induced the outbreak of the countrywide cold wave. Further analysis indicates that in the southward and downward propagation process of the high PV air, the air flow west and north of the high PV center on isentropic surface subsided along the isentropic surface, resulting in rapidly development of Siberian high finally leading to the southward outbreak of the strong cold wave.