Abstract: A field experiment was conducted to study the effects of supplemental irrigation based on soil moisture on the photosynthesis characteristics and enzyme activity of flag leaf using the wheat cultivar Jimai 20. Three irrigation treatments were designed with target soil moisture of 65% (W₆₅), 70% (W₇₀) and 75% (W₇₅) both at jointing and anthesis stages. Zeroirrigation (CK) was used as the control. The results showed that the net photosynthetic rate (P_n) of flag leaf in treatment W₇₀ was dramatically higher than in other treatments from 14 to 21 days after anthesis, as well as sucrose content and sucrose phosphate synthase (SPS) activity. The dry matter mass per area of W₇₀ was higher than that of W₆₅ and CK, and was not significantly different from that of W₇₅. The single stem mass of W₇₀ was higher than that of the other treatments. The activities of superoxide dismutase (SOD) and catalase (CAT) and the soluble protein concentration in flag leaf of W₇₀ were significantly higher than in other treatments from 14 to 28 days after anthesis. The malondialdehyde (MDA) content of W₇₀ was lower than that of W₆₅ and CK, and was not significantly different from that of W₇₅ from 14 to 21 days after anthesis. Grain yields of W₇₀ were 8941.4 and 9125.4 kg·hm^-2 in the 2012-2013 and 2013-2014 wheat growing seasons, showing no significant difference with those of W₇₅, but obviously higher than those of W₆₅ and CK. And the water use efficiency (WUE) of W₇₀ was the highest. Considering grain yield and WUE, maintaining the relative soil water content at 70% by supplemental irrigation both at jointing and anthesis stages was the best treatment.