【Background】The giant ragweed (Ambrosia trifida L.) is an invasive alien weed, not only threatening native biodiversity and ecosystems but also cause a series of allergic reactions in humans. Understanding the factors influencing pollen production would open possibilties to better manage this invasive plant and reduce pollen production. 【Method】The effects of climatic factors on the male inflorescence differentiation of the giant ragweed were analyzed based on the meteorological data from 2008~2010. Five climatic factors, including daily maximum ground temperature, daily minimum ground temperature, daily mean temperature, relative humidity, duration of sunshine were statistically analyzed. 【Result】Male inflorescence completes the whole development in 26 days. During the last period of vegetative growth, lower sunshine and daily mean temperature, higher relative humidity (RH), comparatively large difference in temperature could speed up the differentiation time of the male inflorescence. During early differentiation stage， higher RH is beneficial, although a long time with higher relative humidity is not beneficial. 【Conclusion and significance】Environmental change plays a decisive role in male inflorescence differentiating of giant ragweeds. It will offer reference for determining control time and measures of giant ragweeds.