A cold air index(CAI) is defined as the cold air activity days weighted with respect to the temperature and 24 h temperature change at 850 hPa over Austrilia. The intraseasonal and interannual features of Australian cold air activity are studied using CAI. Results show that the key region of cold air activity is located in the southwest of Australia in May, and its center and influence range display a gradual northward shift in July and August with the main activity region expanding towards the east part of Australia. Australian cold air activity has no obvious interannual difference over the whole winter(JJA), but it has distinct interannual variation in May, June, July and August, respectively.