Floral scent is an important factor in attracting pollinators and repelling florivores. In Phalaenopsis bellina (Orchidaceae), the major floral scent components are monoterpenoids. Previously, we have identified that expression of GERANYL DIPHOSPHATE SYNTHASE (PbGDPS) is highly correlated with monoterpene biosynthesis in Phalaenosis orchids. Here, we showed that both cis- and trans-regulation were present on the GDPS promoters, with trans-regulation playing a key role. To investigate the regulation of floral scent biosynthesis, we compared the transcriptomic data of two Phalaenopsis orchids with contrasting scent phenotypes. Eight transcription factors (TFs) with sequential elevation expressions through floral development stages in P. bellina were identified, and their transcript levels were higher in the scent orchid than the scentless one. Five of these TFs transactivated several structural genes involved in monoterpene biosynthesis pathway to various extent, including PbbHLH4, PbbHLH6, PbbZIP4, PbERF1, and PbNAC1. Ectopic transient expression of these TFs in scentless orchids revealed that terpenoid biosynthesis was all stimulated. PbbHLH4 most profoundly induced the monoterpene phenotype with a 950-fold increase of monoterpenoid production in the scentless orchid. In conclusion, the orchid floral monoterpenes biosynthesis was regulated sequentially and elaborately, and with PbbHLH4 playing a crucial role for monoterpene phenotype.