Abstract: The Northeast China Cold Vortex (NCCV) frequently triggers low temperatures and cold damage during the summer months, causing “Dumb Disasters” in the “Northeast Granary.” Based on the fifth-generation ECMWF reanalysis data, this paper analyzes the key circulation patterns and the mechanisms behind the formation of the NCCV and its low-temperature impacts during early summer (May–June) from 1979 to 2021. The results reveal that two ridges—one over Lake Baikal and the other over the Okhotsk Sea—along with the NCCV, form a typical inverted “Ω” circulation pattern. This configuration facilitates the intrusion of cold air and high potential vorticity from high-latitude regions into Northeast China. Simultaneously, easterly winds from south of the high-pressure system over the Okhotsk Sea further transport cold air into the region. These two cold air intrusions result in abnormal cooling in Northeast China. A “− + −” Rossby wave train, stretching from the eastern European Plain to Lake Baikal, acts as the precursor to the NCCV. This wave train is amplified by the diabatic heating and friction. Subsequently, the Rossby wave mechanism takes over, driving high potential vorticity southward and leading to its accumulation. The East Asian jet barotropically deforms this disturbance to form a deep NCCV. The NCCV weakens significantly as Rossby wave energy disperses downward as well as diabatic heating and friction dissipate. When a quasi-stationary ridge persists north of Lake Baikal, it disrupts the connection between Northeast China and the cold air and high potential vorticity over the high-latitude region. Under these conditions, the NCCV is hardly intensified by the Rossby wave, and its impact on low temperatures in Northeast China becomes negligible.