Water deficit is one of the major limiting photosynthesis of plants in many regions. A comparative study on photosynthetic characteristics between the genetic modified sweet potato transferred with the Cu/Zn SOD and APX genes (TS) and non-GM sweet potato (NS) were carried out with pot experiments. Three controls, namely the favorable soil moisture (75%-80% of field water capacity, FWC), the moderate drought (60%-65% of FWC) and the severe drought (35%-40% of FWC) groups were used. Result showed that light response curves of the TS and NS have no significant difference under the three soil moisture conditions. However, the photosynthetic rates of the TS were all higher than that of the NS. This was especially significant for the severe drought condition, which proved that the transcription of the Cu/Zn SOD and APX genes can help in maintaining a higher photosynthetic capacity under drought stress. To be specific, difference of the net photosynthetic rate (P_max) between TS and NS was not notable under favorable and moderate drought conditions. But the P_max of TS was obviously superior under severe drought stress. The apparent quantum efficiency (φ) of NS declined significantly under moderate drought. Comparatively, the φof TS only decreased slightly. Although theφvalues were both significantly decreased under severe drought stress, TS could still maintain a higher level of φthan NS. This indicated that the transcription of antioxidant enzyme genes in sweet potato has promoted its overall photosynthetic capacity by inhibit the decrement of weak-light energy conversion efficiency under drought stress. The Light compensation point (LCP) would increase; and the light saturation point (LSP), apparent quantum efficiency (φ) and dark respiration rate (R_d) etc. would decrease when the degree of drought rose. Here, drought stress has compelled a comprehensive stomatal closure, reduced the utilization efficiency of light and CO₂, raised the consumption of photosynthates. This is not conducive to the accumulation of anabolites. As the drought intensified, the increment of LCP, photorespiration and the decrement of LSP of TS were all smaller than that of NS. It's possible that the transferred antioxidant enzyme genes have promoted the leaves' bioactivity of the enzyme-recovery-system under drought stress. And strengthen of the osmolytes accumulation has alleviated the membrane lipid peroxidation injuries by drought stress.