In midsummer (July-August) of 2017, the high temperature (HT) frequency in China breaks the record since 1951. For each HT threshold from 35 °C to 40 °C, the frequency is beyond the normal by 2.58–3.46 standard deviations. Specifically, the HT events in southern China make major contributions to the new record, and they demonstrate obvious temporal variations. The strongest episode in southern China appears in mid-to-late July and lasts for nearly 20 days. The westward extension and intensification of the western Pacific subtropical high (WPSH) is usually considered as the leading factor of the HT in southern China. However, the diagnostic analysis in this paper shows that the 2017 HT event does not correspond to the intensification of the whole WPSH but to its western part, and there is a cyclonic circulation anomaly over the western Pacific warm pool, which is closely related to the southward extension of the middle-latitude trough over the east side of Japan. This circulation pattern is favorable for the southward retreat of the WPSH northern boundary over the warm pool, which results in the shape change and even the split of the high. Then the west part of the high is incapable of withdrawing eastward, and it dominates over the southern China steadily, leading to the persistent HT events. This conclusion is further confirmed by the statistical analysis both of the multi-year circulations and the typical HT extreme case in 2013, which could provide reference for the sub-seasonal prediction of HT in southern China.