Flowering is a vital agronomic trait that determines the economic value of most ornamental plants. The flowering time of rose (Rosa spp.) is photoperiod insensitive and is thought to be tightly controlled by light intensity, although the detailed molecular mechanism remains unclear. Here, we showed that rose plants flower later under low-light (LL) intensity than under high-light (HL) intensity, which is mainly related to the stability of PHYTOCHROME-INTERACTING FACTORs (RcPIFs) mediated by OPEN STOMATA 1-Like (RcOST1L) under different light intensity regimes. We determined that HL conditions trigger the rapid phosphorylation of RcPIFs before their degradation. A yeast two-hybrid screen identified the kinase RcOST1L as interacting with RcPIF4. Moreover, RcOST1L positively regulated rose flowering and directly phosphorylated RcPIF4 on serine 198 to promote its degradation under HL conditions. Additionally, phytochrome B (RcphyB) enhanced RcOST1L-mediated phosphorylation of RcPIF4 via interacting with the active phyB-binding motif. RcphyB was activated upon HL and recruited RcOST1L to facilitate its nuclear accumulation, in turn leading to decreased stability of RcPIF4 and flowering acceleration. Our findings illustrate how RcPIF abundance safeguards proper rose flowering under different light intensities, thus uncovering the essential role of RcOST1L in the RcphyB-RcPIF4 module in flowering.