The PhSPL9b gene was cloned to study the role of SPL （SQUAMOSA-promoter binding protein-like） transcription factor in the flowering transformation in petunia. Point mutation of the miR156/157 target site corresponding to the PhSPL9b gene was conducted to obtain rPhSPL9b. The overexpression vectors for 35S∶∶PhSPL9b and 35S∶∶rPhSPL9b were constructed and transformed into petunia and Arabidopsis. Transgenic plants of Arabidopsis overexpressing PhSPL9b or rPhSPL9b and transgenic plants of petunia overexpressing PhSPL9b were obtained. The results showed that overexpression PhSPL9b or rPhSPL9b， especially 35S∶∶rPhSPL9b， significantly reduced the number of rosettes and promoted flowering in Arabidopsis compared with the control （CK）. The phenotype of transgenic Arabidopsis with 35S∶∶rPhSPL9b was more obvious. Overexpression of PhSPL9b significantly promoted flowering in petunia. The results of analyses with RT-PCR and qRT-PCR showed that the expression level of PhSPL9b in transgenic lines with obvious phenotypes was significantly higher than that in the control. The results of transcription activation showed that PhSPL9b was a transcription factor with the function of activating transcription. It is indicated that the PhSPL9b gene in petunia plays an important regulatory role in flowering time， and its function is conserved. At the same time， it may affect plant flowering by transcriptionally activating the expression of downstream genes.