Based on the daily reanalysis data of the Japan Meteorological Agency (JRA55) from 1958 to 2020, the thermal budget characteristics before and after winter onset in North Asia (50°-75°N,80°-140°E) and the external forcing factors that affect winter onset are studied. The result shows that the climatological winter onset in North Asia occurs in the sixty-second pentad. Before the onset, North Asia experienced a process from drastic cooling to slow down. In this process, the diabatic cooling plays a major role, which is partially offset by the warm advection. Based on the linear baroclinic model, the diabatic cooling in the middle and high latitudes of the Eurasian continent is conducive to the formation of westerly and southerly winds in North Asia, resulting in strong local warm advection, and then partially counteracting the cooling effect of the diabatic heating itself. On the interannual time scale, the sea surface temperature anomalies in the Equatorial east-central Pacific and North Atlantic both affect winter onset in North Asia through anomalous wave train on the Eurasian continent, and they show significant signals one month and five months in advance, respectively, which has certain predictive significance. In terms of its impact on winter temperatures, the temperature in some parts of East Asia and southern China is higher (lower) in December in the early (late) year of winter onset in North Asia.