The 8yearold ‘Junzao’ plants were selected in the irrigation area along the Yellow River. With no potassium fertilizer as the control, 300 g potassium sulfate was applied in the flowering stage, young fruit stage, fruit expansion stage and color transformation stage of fruit trees. The changes of fruit quality, yield, leaf photosynthesis and fluorescence parameters of each treatment were measured, and the best potassium application period of ‘Junzao’ in the irrigation area along the Yellow River was determined, which provided a theoretical basis for the formulation of reasonable fertilization measures for jujube trees. The results showed that: (1) compared with the control, the application of potassium fertilizer promoted the photosynthesis and improved the fruit quality of ‘Junzao’ jujube, and the effects were different in different growth stages. And the application of potassium fertilizer in the fruit expansion stage had the most significant effect on improving fruit quality, yield and leaf photosynthesis of Junzao jujube. (2) After applying potassium fertilizer during fruit expansion stage, the vertical diameter, transverse diameter, fruit shape index, weight per fruit and yield per plant of ‘Junzao’ significantly increased by 22.64%, 39.33%, 13.62%, 19.52% and 19.58%, respectively compared with the control. The content of Vitamin (Vc), soluble sugar and fructose contents, the ratio of sugar and acid significantly increased by 37.98%, 22.82%, 31.37% and 29.95% compared with the control, respectively. The leaf chlorophyll content, net photosynthetic rate (P_n), stomatal conductance (G_s), transpiration rate (T_r) significantly increased by 28.91%, 38.44%, 36.75% and 27.25% compared with the control, respectively. The leaf initial fluorescence (F₀), maximum fluorescence (F_m) and electron transfer rate (ETR) significantly increased by 28.18%, 29.15% and 37.75% respectively compared with the control. However, the intercellular CO₂ concentration (C_i) and fruit cracking rate reached the lowest value during the fruit expansion stage, which significantly decreased by 28.34% and 35.50% respectively compared with the control. It was found that the application of 300 g potassium fertilizer to the plant during the fruit expansion period of Junzao could significantly increase the chlorophyll content, photosynthesis efficiency, light energy absorption and electron transfer efficiency of the plant leaves, improve the photosynthetic capacity of the tree, significantly reduce the fruit cracking rate, effectively improve the external and internal quality of the fruit, and significantly increase the yield per plant.