Abstract: Mixed planting of deciduous and evergreen economic tree species can optimize the spatial structure of forest stands and reduce the competition of forest trees. Carya cathayensis and Torreya grandis ‘Merrillii’ are respectively crucial deciduous and evergreen nut-producing tree species in the subtropical mountainous area of Zhejiang Province. This study tested the photosynthetic capacity of leaves and the quality parameters of fruit quality in pure stands and mixed forests of the two species, investigating the photosynthetic and physiological regulation mechanisms of C. cathayensis and T. grandis ‘Merrillii’ fruit development under different planting patterns. The results showed that the net photosynthetic rate, photosynthetic capacity, fruit fresh weight and yield of mixed forest were higher than those in pure forests. In comparison to pure forest plantations, the mixed plantation promoted the seed quality of the two tree species, with C. cathayensis showing an increase in photosynthetic products and protein content, T. grandis ‘Merrillii’ demonstrating more starch synthesis from its photosynthetic products. In summary, the mixed planting of C. cathayensis and T. grandis ‘Merrillii’ may have an optimized spatial structure of forest stands and lower stand competition, exhibiting higher photosynthetic capacity and superior fruit quality. This findings provide a theoretical basis for the mixed-cultivation and growth of C. cathayensis and T. grandis ‘Merrillii’.