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Have the onset dates of regional extreme hot events threatening human health been advancing? Based on the daily maximum temperature data from Chinese metrological stations and reanalysis datasets, we quantified the variations in the onset dates of the first regional extreme hot events (EHE-ODs) and revealed the early-onset characteristics of the regional EHEs in southeastern China. The results show that the EHE-ODs in southeastern China have a significant early-onset trend with an average advanced rate of 7.1 days/decade since 1987. The rate in the middle and lower reaches of the Yangtze River and South China exceeds 10.3 days/decade. The interannual variation in the sea ice over the Barents Sea (SICBS) in March and the snow depth over western Eurasia (SDWEA) in April are closely related to the first regional EHE-ODs in southeastern China, which both provide favorable atmospheric circulation patterns and significant descending motion for the occurrence of regional EHEs. The pathway of the physical process originates from the northern Eurasia propagating southeastward, and spreads to southeastern China, which can enhance the anticyclonic anomaly in southeastern China and favor the occurrence of regional EHEs in southeastern China in the early summer. Therefore, the SICBS in March might be a critical potential predictor for the onset of the regional EHEs in southeastern China. • The onset date of extreme hot events (EHE) in southeastern China has an early-onset trend. • The advanced rate of the early-onset trend of regional EHE exceeds 10.3 days/decade. • Barents Sea ice plays as a precursor for the onset date of EHE in eastern China. • Midlatitude Eurasian atmospheric wave train is related to the early onset of EHE. [ABSTRACT FROM AUTHOR]