Chromatin modifications, including histone acetylation, play essential roles in regulating flowering. The CBP/p300 family HISTONE ACETYLTRANSFERASE 1 (HAC1), which mediates histone acetylation, promotes the process of floral transition; however, the precise mechanism remains largely unclear. Specifically, how HAC1 is involved in the flowering regulatory network and which genes are the direct targets of HAC1 during flowering regulation are still unknown. In this study, we elucidated the critical function of HAC1 in promoting flowering via exerting active epigenetic markers at two key floral integrators, FT and SOC1, thereby regulating their expression to trigger the flowering process. We show that HAC1 physically interacts with CONSTANS (CO) in vivo and in vitro. Chromatin immunoprecipitation results indicate that HAC1 directly binds to the FT and SOC1 loci. Loss of HAC1 impairs CO-mediated transcriptional activation of FT and SOC1 in promoting flowering. Moreover, CO mutation leads to the decreased enrichment of HAC1 at FT and SOC1, indicating that CO recruits HAC1 to FT and SOC1. Finally, HAC1, as well as CO, is required for the elevated histone acetylation level at FT and SOC1. Taken together, our finding reveals that HAC1-mediated histone acetylation boots flowering via a CO-dependent activation of FT and SOC1.