Abstract: The objective of this study was to investigate the effects of photoperiods on the growth, flowering and photosynthetic mechanisms of forced Paeonia lactiflora. Two forced P. lactiflora cultivars Da Fu Gui (DFG) and Tao Hua Fei Xue (THFX) were treated with 3 different photoperiods: 14 h/d, 18 h/d and 9 h/d as control (CK). The results showed that: Compared with CK, 14 h/d photoperiod promoted the growth, chlorophyll accumulation and flowering of both cultivars, gradually increased maximum quantum efficiency of PSⅡ (Fv/Fm) and electron supply of PSⅡ to PSⅠ, and a new equilibrium of electron transport between PSⅡ-PSⅠ-carbon assimilation could be quickly reached. In comparison with CK, 18 h/d photoperiod was disadvantageous for the shoot elongation and significantly inhibited the flowering of DFG. In addition, 18 h/d photoperiod negatively affected electron transport chain at PSI, and broke the balance between PSI-carbon assimilation for both cultivars. The photosynthetic electron transport chain of two P. lactiflora cultivars responded differently to photoperiods. DFG had higher electron transport activity than THFX, and under 14 h/d photoperiod, dynamic changes of PSⅡ-PSI-carbon assimilation of DFG could reach a new dynamic equilibrium faster than THFX and keep the balance, therefore, DFG had a stronger photoperiod adaptability than THFX.