In the late spring and early summer of 2021, many areas in China experienced severe convections, causing heavy casualties and property losses, especially in southern China (25.0°~32.5°N, 112.5°~122.5°E) from 10 to 16 May. Through analyzing the features of the atmospheric circulation anomalies from 10 to 16 May 2021, as well as the associated dynamical, thermal and moisture conditions, the study explores the causes of frequent severe convections in southern China from the perspective of large-scale environment. The results show that the frequent severe convections are closely related to large-scale circulation anomalies. At 500 hPa over Eurasia, the geopotential height anomalies exhibit a northwest-southeast oriented wave train pattern with a “+,-,+” distribution. Along this wave train, the Ural Mountains and the coastal areas of East Asia are dominated by positive height anomalies, while the areas from Lake Baikal to Balkhash Lake are occupied by negative height anomalies. The positive anomalies along the coastal areas of East Asia correspond to an intensified and northward shifted western Pacific subtropical high (shorted as WPSH). On the one hand, the anomalous southwest warm and moist airflow to the north of WPSH is beneficial to the development of ascending motion as well as the establishment of unstable stratification in southern China. On the other hand, the pressure decrease resulting from the anomalous warm advection is favorable for the frequent occurrence of low-level convergence systems in southern China, which provides triggering conditions for severe convections. In addition, the strong vertical wind shear formed between high-and low-level jets is also one of the favorable dynamic conditions for the development of severe convections. Further analysis indicates that there is a southeastward propagating anomalous wave activity flux, which originates from the high latitudes of Eurasia, suggesting that the wave train-like circulation anomaly is closely associated with the dispersion of the Rossby wave energy. Therefore, the atmospheric dynamical process in mid-high latitudes plays an important role in the frequent severe convections in southern China.