Abstract: The Oyashio and its extension exhibit high SST variability, with frequent occurrences of high-intensity marine heatwaves. However, previous research mainly focuses on summer, with limited studies on the colder seasons. In this study, using satellite observation data and model reanalysis results, we systematically analyzed the changes in marine heatwaves during the winter and spring seasons in the Oyashio and its extension, as well as potential influencing factors. From 1992 to 2021, marine heatwave index (mean intensity, frequency, duration, cumulative intensity) in the region demonstrated a clear upward trend, with respective values of 0.39 ℃/(10 a), 0.38 times/(10 a), 3.05 d/(10 a), and 8.09 (℃·d)/(10 a). The upward trend correlates well with the accumulated anticyclonic eddy kinetic energy. In the spring of 2021, the Oyashio and its extension experienced the strongest recorded marine heatwave event. The mixed layer heat budget analysis shows that the event was primarily due to the horizontal advection anomaly caused by the anomalous eddy term. Anomalous anticyclonic eddies, either detaching from the Kuroshio Extension carrying warm water northwards or intensifying in strength, led to the occurrence of the heatwave.