Abstract: Effects of spermidine (Spd) and arbuscular mycorrhizal fungi (AMF) on the growth and photosynthesis of cucumber (Cucumis sativus L., cv. Jinchun 2) seedlings, fruit yield and quality, microorganism and enzymatic activity in rhizosphere were investigated. The results showed that combined Spd and AMF addition significantly promoted the growth of cucumber seedlings, increased the root activity, fruit production and quality, enhanced nutrients absorption of seedlings. Net photosynthetic rate, actual photochemical efficiency, light quantum efficiency, carboxylation efficiency and light respiration rate of cucumber seedlings were increased by combined addition of Spd and AMF. The abundance of bacteria and actinomycetes in the rhizosphere of cucumber seedlings were increased, but that of fungi was decreased. The activities of enzymes, including sucrase, neutral phosphatase, catalase, and urease, were enhanced. These results suggested that the light utilization efficiency of cucumber seedlings was improved by combined addition of Spd and AMF. The growth substrate has been changed from a low-fertility type caused by fungi to a high-fertility one caused by bacteria. The decomposition and transformation of organic phosphorus and nitrogen in substrate were increased by combined addition of Spd and AMF, and thus could provide more N and P for the growth and development of cucumber, which further led to enhanced seedling growth, higher yield and quality of fruits. Moreover, Spd could increase the infection rate of AMF in cucumber roots, and together they had a synergistic effect on the growth of cucumber. Our results suggest that it is an effective method to enhance the infection rate of AMF by using Spd.