Abstract: The NECV (Northeast cold vortex) is a key component of the East Asian mid- to high-latitude atmospheric circulation, significantly impacting temperature and precipitation in Northeast and North China. In this study, the ERA5 daily reanalysis dataset from the European Centre for Medium-Range Weather Forecasts was used to objectively identify the boreal winter NECV over 42 years (1979–2020) and to explore its basic climatology, including its life cycle, intensity, spatial distribution, and number of active days. Furthermore, an NECV index (NECVI) was defined to analyze the multiscale features of NECV activity. Subsequently, SVD (Singular Value Decomposition) and regression analyses were performed based on the NECVI to examine the intraseasonal influence of the NECV on winter temperatures in Northeast and North China (39°N–54°N, 115°E–135°E). Results indicated that the study period (42 years) witnessed 266 boreal winter NECV events within the study area (35°N–60°N, 115°E–145°E). The NECV was mainly located in the northern region (45°N–60°N, 115°E–145°E), with an average life cycle of 4.3 d. The intensity of the boreal winter NECV and NECVI indicated reversed NECV strengths in January and February of the initial and final 21 years of the study. Specifically, this reversal was observed when comparing the two halves of the study period: before 2000 (1979–2000), the NECV intensity was stronger in January and weaker in February, whereas after 2000 (2000–2020), the intensity was weaker in January and stronger in February. Furthermore, the NECV activity exhibited significant interannual variability (with a change period of 2–3 years for the NECV), whereas the boreal winter NECV activity exhibited an intraseasonal oscillation period of 10–40 d. Additionally, time-lag regression revealed the dynamical mechanism underlying the development of low-frequency NECV: during the low-frequency NECV development phase, two low-frequency wave trains were observed in the mid-to-high latitudes of the Northern Hemisphere. At day 0 (peak development), a zonal low-frequency wave train exhibiting a “+, −, +” configuration was identified in the upper atmosphere, extending from the Ural Mountains across Northeast Asia to the areas east of Japan. Simultaneously, a transition pattern resembling a shift in the Arctic Oscillation from a positive to a negative phase was identified between the high-latitude regions of East Asia and the polar regions. This pattern established a low-frequency double-blocking structure, with blocking features over the Ural Mountains (upstream) and over the Sea of Okhotsk/western North Pacific (downstream). This stable configuration enabled the low-frequency NECV to persist, resulting in sustained cooling in Northeast and North China. Notably, the occurrence of extremely low temperatures in the Northeast and North China regions, which occurred at low frequency, lagged the low-frequency NECV by about 3–5 d.