The linear trend analysis, Mann-Kendall test, and Morlet wavelet analysis methods are applied to analyze anomaly and cumulative anomaly of sea level pressure derived from the NCEP/NCAR reanalysis over the Northwest Pacific (20°N-50°N). The seasonal transition time of sea level pressure over this region in the most recent 35 years (1979-2014) are revealed with a focus on its long time trend and periodic oscillation on multiple time scales. The results show that the sea level pressure experiences two seasonal transitions each year: The transition from winter status to summer status that takes place at about the 20th pentad, and the transition from summer status to winter status that occurs at about the 51th pentad. In addition, the seasonal transition time of the seal level pressure field varies with latitudes and longitudes. Using the trend analysis method, we find that the transition time of seal level pressure from winter status to summer status has significant interannual variability. In the most recent 35 years, the seasonal transition time from winter status to summer status trends to become early with a changing rate of about -0.33 pentad/10 a; the transition time from summer status to winter status has postponed by about 0.25 pentad/10 a. The result of Mann-Kendall test indicates that the transition time of sea level pressure from winter status to summer status experienced an abrupt change in 1997-1998, but there is no similar abrupt change in the transition time from summer status to winter status. Finally, using Morlet wavelet analysis and significance test for global wavelet power spectrum, we conclude that the transition time from winter status to summer status has a significant 15-years periodic oscillation and the transition time from summer status to winter status fluctuates primarily with an 8-year cycle.