Abstract: Using ERA5 reanalysis data, this study investigates the timing of the Northern Hemisphere stratospheric polar vortex (NSPVO) onset during the seasonal transition of atmospheric circulation from summer to winter over 1950–2022. This study examines the circulation differences and tropospheric precursor signals between early- and late-onset NSPVO events, as well as the physical mechanisms underlying these variations. Results indicate that the mean NSPVO onset date at 10 hPa is August 27, with the maximum range of the establishment spanning 24 days. The average time discrepancy between the 17 early-onset and 13 late-onset NSPVO years is 12 days. In the early-onset NSPVO years, the Northern Hemisphere baroclinic annular mode (NBAM) in the troposphere at middle and high latitudes anomalously strengthens starting from the second week before NSPVO. Correspondingly, an anomalously enhanced Rossby wave train appears over the Asia–North Pacific tropospheric region, with Rossby wave 4 dominating the anomalous upward propagation of wave activity flux (WAF) and promoting the enhancement of the zonal-mean NBAM during this stage. Under the influence of this Rossby wave, the eddy westward momentum flux anomalously converges outside the polar region, inducing an acceleration of the westerly wind. This acceleration process, with a maximum value exceeding 0.4 m s⁻¹ d⁻¹, propagates from outside the polar region toward the pole. However, this influence is mainly confined to the stratosphere and troposphere below 100 hPa. From a thermodynamic perspective, the eddy heat flux anomalously converges and heats outside the polar region and above 100 hPa, reaching a maximum of 0.1 ℃ d⁻¹, and it develops from lower to upper levels. The resulting increase in the temperature gradient between the inside and outside of the polar region drives the westerly wind anomalies into the middle and upper stratosphere through the thermal wind mechanism. Under the combined effect of the two aspects, in the week before NSPVO, the stratospheric westerly wind below 10 hPa increases by more than 2.5 m s⁻¹ near 70°N, leading to early-onset NSPVO. However, if the regional Rossby wave is abnormally weakened in the early stage, NSPVO will be delayed.