Abstract: Based on the daily observation data during 1979-2013, the characteristics of internal oscillation in summer extreme high temperature in southern of the Yangtze river were further discussed. By using Butterworth filter and power spectrum methods, results show that: From the middle of July to the middle of August, the high value center moves into southern of the Yangtze river(110°-125°E,22.5°N-30°N) and averaged hot days beyond 95% threshold show oscillation feature. Result of Power spectrum indicates daily maximum temperature presents the characteristics of intraseasonal variation in 10-25d. The impact system is not directly related to the subtropical high in the typical low frequency year(1989 and 1993). On the middle and high troposphere, the geopotential height presents 2 to 3 waves with the obvious intraseasonal oscillation features(10-15 day and 20-25day). The motion and propagation of the wave may be related to the low frequency variation of temperature.