With the Li-6400 system we measured the diurnal changes of photosynthetic characteristic of leaves of jujube and its relationships with water use efficiency. The results showed that the diurnal changes of net photosynthetic rate and transpiration rate were single-apex curves, the peak value of net photosynthetic rate was present at 14:00, the peak value of transpiration rate was present at 16:00. Correlation analysis indicated that net photosynthetic rate was extremely significant positively correlated to transpiration rate（p＜0.01） and stomatal conductance（p＜0.01）. Photosynthetic mechanism of jujube was studied from photosynthetic characteristics and water use efficiency and this provided theoretical basis for high quality and yield of jujube.