Abstract: A study was conducted to investigate the seasonal dynamics of photosynthesis and the underlying regulating factors for three typical species, i.e., Styrax benzoinoides Craib, Ligustrum lucidum, and Cinnamomum saxatile, in Southwest Karst, China. Photosynthesis was measured using LI-6400 portable pothosynthesis system. Net photosynthetic rates (Pn) of Ligustrum lucidum and Cinnamomum saxatile were higher in April and July, while lower in January and October, while a clear seasonal pattern of Pn for Styrax benzoinoides Craib was not observed. All the three species were found to exhibit the phenomenon of ‘noon break’, but each had different curves. Different factors affected the Pn of the three species in different season. The photosynthetic active radiation (PAR) and transpiration rate (Tr) were the main factors in January and October. The factors regulating Pn for the three species varied in April, with the intercellular CO₂ concentration (Ci) and PAR being the main factors for Ligustrum lucidum, and PAR and Tr being the main factors for Cinnamomum saxatile. Netherless the effects of all the factor on Styrax benzoinoides Craib was not significant. In July the main factors were stomata conductance (Gs) and Ci. PAR was the main factor affecting Pn of the three species in different season. We concluded that Ligustrum lucidum, and Cinnamomum saxatile could be more adaptive to the environments of Karst, while Styrax benzoinoides Craib was less adaptive to the environments of Karst in the Southwest China.