Fingered Citrons (Citrus medica var. sarcodactylis Swingle) is a chilling-sensitive ornament fruit plant. The chilling stress greatly affects its growth and yield. In this study, we studied the changes of photosynthetic physiology under a simulated chilling condition which happens frequently in mid-Zhejiang area. The results showed temperature below 15℃ significantly decreased the net photosynthetic rate, stomatal conductance and increased the intercellular CO2 concentrations. The 10℃ was the inflection point where the Fv/Fm began to decline rapidly and Fo began to increase. However, the 15℃ decreased the Fv/Fm significantly if the treatment length was postponed to 72h. Chilling stress also led to declines of carboxylation efficiency, the maximum photosynthetic rate, and the PFD when the photoinhibition was observed. In contrast to the control, the chilling treatment generally led to a rise of relative electric conductivity (REC) in leaves and declines of SOD, POD and CAT activities. In brief, short term chilling stress primarily weakened the activity of key enzyme in photosynthetic carbon fixation, such as Rubisco, caused an accumulation of reactive oxygen species, and then led to photoinhibition and a decline in photosynthetic rate.