Based on the daily surface temperature data from National Climate Center of China,three summertime high temperature (SHT) indices: SHT days,SHT intensity and daily maximum temperature are defined to depict the summertime high temperature (SHT) in Yangtze River Delta (YRD) from different perspectives. On this basis,the mutations of the SHT and the before and after comparison of the spatial and temporal characteristics of SHT are detected over the period 1961-2015. The result shows that there exits an abrupt mutation in SHT around 2000. As compared to the SHT before 2000,more SHT days and higher SHT intensity are found after 2000. The analysis of Empirical Orthogonal Function (EOF) shows that the explained variances of the first three modes have an increase,indicating a more concentrate SHT distribution. Compared with the first three modes before 2000,a similar distribution of EOF1,a non-zonal distribution for EOF2 and a weak-locality distribution for EOF3 are found after 2000. The areas with SHT are enlarged after 2000. The areas with SHT exceeding 20 days have a northward expansion to around 32°N and an eastward expansion to the coast of Zhejiang province. Moreover,the areas with SHT exceeding 30 days cover the whole region to the south of 32° N and west of 121° E,and the maximum SHT days is up to almost 50 days. The SHT days have an apparent increase from 10 ~ 20℃·d before 2000 to 20 ~ 50℃ ·d after 2000,which is most significant around 30 ~ 31°N. After 2000,it has an evident increase for SHT days and comparatively weak increase for SHT intensity in middle and south of Zhejiang province,and both evident increase for SHT days and intensity in east Anhui,north Zhejiang,and Shanghai.