【Background】Ambrosia trifida L. is a harmful weed all over the world, causing harm to human health, farming as well as animal husbandry. 【Method】The contents of three endogenous hormones gibberellic acid (GAs), indoles3acetic acid (IAA), and abscisic acid (ABA) were examined by highperformance liquid chromatography (HPLC). 【Result】During the flower bud differentiation, and during the middle of flower bud differentiation, the content of GAs lowered and increased at the later stage. The content of IAA increased constantly. The amount of ABA increased fast at the later stage. 【Conclusion and significance】ABA seemed to be the most important hormon during flower bud differentiation. The study will provide reference for chemical control of the ragweed.