Abstract: In this study, “Red Globe” grape (Vitis vinifera L.), a great breed of Eurasian Grape which was planted widely in recent years in the South of China, was studied for the responses of photosynthetic characteristics in “Red Globe” leaves to different magnesium (Mg) concentration (0 mmol/L、1 mmol/L、2 mmol/L、3 mmol/L、4 mmol/L、5 mmol/L) by the sandy culture method. The results indicated that when the concentration of Mg were 0 mmol/L and 1 mmol/L, the chloroplasts structure were damaged, the size and number of chloroplasts decreased, and the lamellae of the grana and stroma were disintegrated gradually, the number of grana lamella was decreased, in contrast, no different was observed under other treatments. The content of chlorophyll a (Chl a) and chlorophyll b (Chl b), the photosynthetic rate (Pn), transpiration rate (Tr), stomatal conductance (Gs) and intercellular CO2 concentration (Ci) were the highest under 3 mmol/L. There were non-linear positive correlations between Mg application rates and photosynthetic properties. These analysis suggested that in South Mg deficiency region moderately fertilize Mg (about 3 mmol/L) could significantly stabilize and integrate ultrastructure of chloroplasts, increase the contents of chlorophyll, and improve photosynthesis.