Abstract: This study utilizes daily maximum temperature data (CN05.1) in China from 1961 to 2017 to reveal the dominant modes of interannual variation in the number of summer extremely-high-temperature days (EHTD) through empirical orthogonal function analysis. It further explores the key factors and underlying physical mechanisms leading to each identified mode. The results show that: (1) The first mode is characterized by a zonal distribution across China and is closely associated with the Arctic Oscillation (AO). A Rossby wave train originating from northern Europe and moving southward increases the zonal high anomaly across China when the AO exhibits a positive phase. (2) The second mode shows a meridional dipole pattern, mainly influenced by the Polar–Eurasian teleconnection wave train spreading from the North Atlantic Ocean to East Asia. Further, sea surface temperature anomalies in the western tropical Pacific enhance the local Hadley cell, placing southern China under high-pressure systems and northern China under low-pressure systems. The increase in EHTDs observed in the first two modes is attributed to increased incident solar radiation caused by reduced precipitation owing to local high-pressure anomalies. (3) The distribution of the third mode is concentrated in the Tibetan Plateau, which is mainly influenced by a zonal wave train propagating downstream from the Mediterranean Sea. The circulation anomalies corresponding to the wave train cause the divergence of water vapor and weakening of upward motion, decreasing cloud cover and increasing downward cloudy-sky shortwave radiation. Simultaneously, it causes atmospheric warming, increasing downward clear-sky longwave radiation. Both effects create favorable conditions for an increase in the number of EHTDs. The results of this study deepen the understanding of the characteristics of extremely high temperatures during summer in China, offering a theoretical reference for the seasonal prediction of EHTDs in the future.