The effects of red soil moisture on the chlorophyll content, net photosynthetic rate (Pn), intercellular CO2 concentration (Ci), difference of CO2 concentrations(△Ca) passing in and out CO2 chamber, stomatal conductance (Gs), transpiration rate (Tr), and carboxylation efficiency (CE) of orange leaves under different soil relative moisture treatments were studied. The results showed that when the soil relative moisture was 75%, the contents of chlorophyll a, chlorophyll b and carotinoid of orange leaves reached the maximum, and there was a positive correlation between Tr and soil relative moisture ( R2 = 0.9625** ); but when the soil relative moisture was lower than 75%, there were positive correlations between Pn, △Ca, CE and soil moisture, this indicated that no relation between Pn and the transpiration of orange leaves when the soil relative moisture over 75%. There was negative correlation between Pn and Ci. There were significant differences between Gs at the treatment of soil moisture equal to 30% and those of other treatments, this indicated that Pn change of orange leaves was mainly caused by non-stomatic inhibition when soil relative moisture was lower than 30%, but mainly caused by stomatic factors when the soil relative moisture over 30%.