Abstract: The decrease in photosynthetic capacity caused by iron deficiency chlorosis is a key limiting factor for the improvement of peanut yield and quality in saline-alkali soil. To explore the effects of quinoa-peanut intercropping on iron nutrition, photosynthetic characteristics, and belowground response mechanisms of peanut plants in saline-alkali soil, a field experiment combined with root separation methods was conducted. The field experiment was designed with four treatments: peanut monocropping (MP), quinoa-peanut intercropping without separation (IPW), quinoa-peanut intercropping with nylon net separation (IPN), and quinoa-peanut intercropping with plastic sheet separation (IPP). The effects of different intercropping treatments on iron uptake and utilization, photosynthetic characteristics, soil physical and chemical properties, root morphology, and yield of peanut plants were analyzed. The results showed that compared with MP, IPW significantly increased the iron content and uptake in peanut plant organs at the seedling and pod-setting stages, enhanced the content of chlorophyll and carotenoids, reduced the leaf yellowing rate, improved the net photosynthetic rate, and increased dry matter accumulation in the late growth stage, thus ensuring the pod yield of peanut plants. IPN improved the iron uptake and photosynthetic characteristics of peanut plants to a certain extent at the pod-setting stage, with weaker effect than IPW. IPP did not show any improvement in iron uptake or photosynthetic characteristics. At the seedling and pod-setting stages, IPW reduced soil electrical conductivity by 28.85% and 14.27%, increased soil available iron content by 7.09% and 9.62%, and elevated soil organic matter content by 17.28% and 29.52%, respectively. The root growth and morphological development of peanut plants at the pod-setting stage under IPW were better than those under MP. Correlation analysis showed that peanut iron uptake and peanut yield were negatively correlated with soil electrical conductivity and significantly positively correlated with soil available iron content and root morphological development indicators such as total root length and root surface area. To summarize, IPW can improve the iron nutrition and enhance the photosynthetic performance and dry matter accumulation of peanut plants in saline-alkali soils. The reduction in soil electrical conductivity, increase in available iron content, and optimization of root morphological structure are the underground response strategies of intercropping quinoa to improve iron nutrition of peanut plants in saline-alkali soils.