Recent increasing wildfire activities in Western US (WUS) have raised widespread public concerns. Whether this increase is related to natural variability or anthropogenic warming is of particular interest in the research community. We observed that vapor pressure deficit (VPD), an important fire weather risk index, in the warm season (May to September) of WUS, has increased significantly since 1979, with a positive trend of 0.47 hPa per decade, and this trend is also significant but smaller for VPD in large fire days; trend analysis of saturated and actual vapor pressure suggests that direct warming and drying contributes to about 70% and 30%, respectively, to the positive VPD trend. This increase of VPD and warming for such a short period cannot be simply linked to anthropogenic factors as it can also be affected by atmospheric internal variability. Using a flow analogue approach, we estimated circulation changes contribute to about half (52%) of the observed VPD trend over WUS. Assuming circulation changes represent the upper limit of internal variability, our results imply that anthropogenic warming must explain at least the other half of the observed VPD trend. Although it’s still not clear how much of circulation changes could be related to anthropogenic warming from pure observation data, the large contribution from anthropogenic warming indirectly inferred in this study highlight the need to better understand and tackle the increasing wildfire and its impacts under the global warming background.