Abstract: Diurnal dynamics of photosynthetic parameters of Viola baoshanensis and V. yedoensis were investigated based on pulse amplitude modulated (PAM). Y (Effective quantum yield) of the two species decreased at midday, indicating that photoinhibition occurred. ETR (electron transport rate) of the two species increased very quickly in morning, and kept at a high level, then decreased. There were difference variation patterns between the two viola species for NPQ， rETR，Pm， Ik and ， and few significant relationships between leaf temperature, PAR and Chlorophyll fluorescence parameters for V. baoshanensis except between PAR and rETR. For V. yedoensi, the significant negative relationships were found between leaf temperature, PAR and Y, significant positive relationships between leaf temperature, PAR and NPQ, and a significant positive relationship between PAR and NPQ. The above results showed that excess light energy dissipated via the xanthophyll cycle and large proportion of electrons allocated to photorespiration for V. yedoensi played roles in protecting the photosynthetic apparatuses. For V. baoshanensis, preventing photodamage was mainly through large proportion of electrons allocated to photorespiration.