Abstract: The structure evolution and explosive development mechanisms of the Yellow Sea and Bohai Sea cyclone causing the extreme snow in Northeast China from 7 November to 9 November in 2021 are analyzed with high-resolution observations and the fifth-generation European Centre for Medium-Range Weather Forecasts (ECMWF) atmospheric reanalysis data (ERA5) with a 0.25° by 0.25° spatial resolution. Results show that the extreme snowstorm occurs under the background of high-altitude cold vortex collocated with surface cyclone. After the formation of the surface cyclone in the Yellow Sea, it strengthens rapidly and moves northward along the eastern part of Northeast China. The snowfall area is mainly distributed on the west side of the cyclone, and the snowfall intensity is closely related to the occurrence and development of the surface cyclone. Its explosive development stage corresponds to the strongest period of the extreme blizzard process. The Yellow Sea and Bohai Sea cyclone is generated from a ground inverted trough which gradually strengthens with eastward shift into the sea. During its explosive developing and occluding stages, the leaf cloud system evolved into hook comma cloud system and vortex cloud system. The horizontal structure shows frontal fracture and the warm front back bending and wrapping, while the vertical structure shows high-altitude frontal fracture, the emergence of dry and warm center, the formation of neutral occluded front, and deep low value system from inclined vortex column. Wave activity flux analysis shows that the ridge in Siberian, the trough in North China and the ridge in Northeast China at 500 hPa devote to Rossby wave train. With the continuous eastward movement and the wave energy dispersion downstream of the positive anomaly center in the upper reaches of Siberia, the wave activity flux from the northwest in the North China trough is rapidly enhanced, and therefore the cold vortex enhances rapidly. The sharp enhancement of vorticity factor over surface cyclones is beneficial to the explosive development of cyclones. The potential vorticity diagnosis on the isobaric surface shows that the abnormal area of positive potential vorticity gradually approaches and superimposes on the middle and low-level system, with the continuous southward development and downward propagation of stratospheric high-level vorticity along the isentropic surface, resulting in the rapid development and downward extension of middle-level cold vorticity and thereby the explosive enhancement of surface cyclones. In addition, the slow downward propagation of potential vorticity is also conducive to the maintenance of occluding stage in the frontal cyclone.