The pathway of the East Asian winter monsoon (EAWM) that usually leads to the out-of-phase pattern of surface air temperature between northern and southern East Asia is an important feature in the variability of the EAWM besides its strength. Using the European Centre for Medium-Range Weather Forecasts 40-year (ERA40) reanalysis dataset, this study investigates the pathway-related stationary wave activity to explore the mechanism of the interannual variations in the EAWM pathway. It reveals that when the southern pathway of the EAWM is strong, the phase of the climatological stationary wave tends to be shifted westward significantly in both the horizontal and vertical directions by an anomalous wavenumber 2 pattern at mid-latitudes, whereas the changes are relatively small in the subtropics. The horizontal changes in the stationary wave phase facilitate a north-south-oriented East Asian trough in the middle troposphere that eventually produces the strong southern pathway of the EAWM. The vertical changes in the stationary wave, in contrast, feature a westward-tilted phase line with height over the North Pacific, indicating enhanced upward propagation of waves into the stratosphere. This result suggests that the phase of stationary waves at mid-latitudes dominate the interannual variations in the EAWM pathway. Moreover, it supports our previous interpretation of the possible role of the North Pacific sea surface temperature (SST) in the EAWM pathway variability. It also implies that the excitation of anomalous mid-latitude stationary waves may be the key in the response of the EAWM pathway to the North Pacific SST.