All-season warming is thought to advance snow melt and delay snow accumulation; coupled with warming-induced drought stress, this could extend both the beginning and end of the fire season. Using the updated fire data of 1967-2018, we found an asymmetrical expansion of the lightning-ignited fire season in the boreal forest of Northeast China. The lightning-ignited fires have not advanced into the early fire season (May-June) but have largely extended into the snowless late fire season since the late 1990s. The length of the lightning-ignited fire season increased by 10.9 days per decade from 1968-2018 (P<0.01), and the mean annual number of lightning-ignited fires has increased from 9.8 to 29.2 times/year since 1998. Despite significant warming, the Julian days of snow melt have changed little, which has prohibited the fire season from advancing into early spring. The expansion of lightning-ignited fires from July-September was associated with warming-induced evapotranspiration and the soil/fuel aridity increase.