Abstract: Using ground-based ozone (O3) measurements, the ERA5 reanalysis data from the European Centre for Medium-Range Weather Forecasts and the WRF-Chem numerical model, this study investigated the impact and mechanisms of wildfires in eastern Siberia on surface O3 anomalies in Northeast China in July 2014. The results indicate that: (1) Wildfire smoke from eastern Siberia can reach and accumulate in Northeast China. This is the result of the interaction among three key weather systems: the extratropical cyclone moving eastward in the westerlies at mid-high latitudes, the high-pressure ridge on its west side, and the northward-moving Typhoon Matmo. The northerly wind on the western flank of the extratropical cyclone transports wildfire smoke through the free atmosphere southward. The downward flow ahead of the high-pressure ridge brings the smoke into the near-surface layer over Northeast China. The airflow along the northern edge of the northward-moving Typhoon Matmo inhibits the eastward transport of smoke, leading to its retention and accumulation in Northeast China. (2) Affected by the transport of upstream wildfire smoke, various locations in Northeast China successively experienced surface O3 pollution events from 25 to 29 July. The positive anomaly of O3 concentration reached the maximum during the daytime on 27 July. The comparison experiments of WRF-Chem with and without wildfires confirmed that, with other conditions unchanged, wildfire smoke can increase the local O3 concentration in Northeast China by up to 40%. (3) The atmospheric composition and reaction rates in the comparison experiments show that the abnormal increase in O3 concentration during the daytime on 25 and 26 July is primarily attributed to the increase in the concentration of peroxy radicals (HO2) induced by the high concentration of carbon monoxide (CO) transported by wildfires. When the O3 concentration reaches its peak during the daytime on 27 July, it is not only associated with the maximum increase in CO concentration but also related to the abnormal increase in the concentrations of formaldehyde (HCHO) that are secondarily generated in the wildfire smoke and related HO2.