It was accurately predicted that in the flood season of 2020, the overall characteristics of “China’s generally poor climate condition, more extreme weather and climate events” and “more serious flooding than drought”. The predictions of abundant precipitation in the middle and lower reaches of the Yangtze River Valley, the middle and upper reaches of the Yellow River Valley, the Haihe River Valley, and the Songhuajiang River Valley, and the deficit precipitation in the Liaohe River Basin, are consistent with observations. The subseasonal monsoon rainy season processes, including the earlier start of South China pre-flood season, the earlier onset and later retreat of Meiyu in Yangtze-Huaihe River Valley, and the later start of North China rainy season than normal, were also well predicted in the climate operation. However, we underestimated the abnormal degree of excessive precipitation in the middle and lower reaches of the Yangtze River, and did not accurately predict the precipitation anomaly in the western reach of the Huaihe River, the Hanshui River, and the Sichuan Basin. The predictions of the surface air temperature are in line with the observations which was warmer than normal in most of China and more high-temperature days in South China. Moreover, we had a good grasp of the tropical cyclone frequency, tracks, and active/inactive periods over the Northwest Pacific and the South China Sea in 2020. For the diagnostic analyses, the weak sea surface temperature (SST) warming in the central and eastern Pacific in pre-winter plus continuous warming in the tropical Indian Ocean were considered to be important predictors. Under the influence of the tropical SST anomalies, the western Pacific subtropical high (WPSH) tended to be stronger and further westward, and anomalous anticyclone dominated the Philippine Sea. Nevertherless, the degree of abnormal warming in the tropical Indian Ocean SST and its impact on the precipitation in the middle and lower reaches of the Yangtze-Huaihe Rivers were underestimated, which is a significant deviation in the prediction. The Climate System Model of the National Climate Centre (NCC_CSM1.1(m)) had a good performance overall in predicting the observation of more precipitation in eastern China, which was mainly related to the accurate prediction of the spatial distribution patterns of atmospheric circulation systems over the tropical and subtropical regions in summer by dynamic models. The subseasonal circulation variations, however, were not well captured, including the double blocking high circulation with “two ridges and one trough” in the mid and high latitudes of Eurasia region in June and July, the persistent southward position of WPSH ridgeline in July, with obviously late subseasonal process.