Using multi-scale analysis, several indicators of cold air activities during the East Asian summer monsoon (EASM) are compared, and the results show that potential vorticity (PV) is the optimal indicator in tracing cold air activities during EASM. The further study shows that it is best that the low-frequency oscillations of PV (20－80 day and 40－80 day oscillations) represent cold air activities. Then, the source and role of cold air during EASM are studied by use of the indicator, and the process and mechanism of interactions between EASM system and extratropical and mid-upper level systems during the Meiyu period are analyzed in detail. The conclusions are as follows: the two high PV pools are the main sources. One exists in the extratropical low stratosphere and tropopause, and the other exists in the low troposphere over the east of the Asian continent. To the intrusion paths, different rainy seasons show different features: during the pre-flood season in South China, the cold air intrudes twice respectively from the northeast at mid-lower levels and the high PV pool of the tropopause; during the Meiyu period, the cold air origins mainly from the high PV pool in the extratropical low stratosphere and tropopause during the onset of Meiyu, and from the high PV pool in the low troposphere over the Sea of Okhotsk during the mid-late period of Meiyu; during the rainy season in North China the cold air origins chiefly from the upper troposphere. Inclined isentropic surfaces are the areas where vertical vorticity grows quickly most easily, and the paths along which the upper cold air intrudes. Each rainy season over eastern China in summer is likely the reflection of the intraseasonal oscillations of the interactions between the EASM system and extratropics, especially the low-frequency part playing a more important role. Finally, the sources and characteristics are sketched.