Overexpression of BpAP1 could cause early flowering in birch. BpAP1 affected the expression of many flowering-related unigenes and diterpenoid biosynthesis in transgenic birch, and BpPI was a putative target gene of BpAP1. APETALA1 (AP1) is an MADS-box transcription factor that is involved in the flowering process in plants and has been a focus of genetic studies examining flower development. Here, we carried out transcriptome analysis of birch (Betula platyphylla Suk.), including BpAP1 overexpression lines, BpAP1 suppression lines, and non-transgenic line (NT). Compared with NT, we detected 8302 and 7813 differentially expressed unigenes in 35S::BpAP1 and 35S::BpAP1RNAi transgenic lines, respectively. Overexpression and suppression of BpAP1 in birch affected diterpenoid biosynthesis and altered expression of many flowering-related unigenes. Moreover, combining information from the RNA-seq database and the birch genome, we predicted downstream target genes of BpAP1. Among the 166 putative target genes of BpAP1, there was a positive correlation between BpAP1 and BpPI. These results provide references for further examining the relationship between BpAP1 and its target genes, and reveal that BpAP1 functions as a transcription regulator in birch.