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Water is not only a limiting factor, but also one of the most important mediators of grape berry compositions in semi-arid land. Differences in superoxide dismutase (SOD), catalase (CAT), proline (Pro), malondialdehyde (MDA), and wine grape berry composition were investigated under water stress with drip irrigation. Merlot vines were proposed to the three levels of irrigation threshold (70-75%, 6065% and 50-55% of field capacity (FC)) at the stage of germination, flowering and enlargement. The sweet taste and the favorable aromatic flavor were improved significantly from the germination to the enlargement stage to which slight water stress (6065% of FC) was applied, wherein an optimal berry composition was achieved at the flowering stage which was mostly responsible for a positive relationship with high SOD activity. Such as, sugar-acid ratio, total sugar, tannins, total phenols, and anthocyanins of berries increased by 183.85%, 47.79%,66.67%, 100%, 57.52% in comparison to normal irrigation threshold (70-75% of FC), respectively. When Merlot vines were stressed at 50-55% of FC from germination to flowering stage, berry composition, diameter and cluster mass were higher than in normal irrigation (70-75% of FC), resulted from the linear positively correlations between berry diameter, dry matter and Pro content, activities of SOD, CAT observed.