The microclimatic effect of intercropping walnut (Juglans regia L.) and plum (Prunus salicina) with corn (Zea mays) and soybean (Glycine max) was studied in an agroforestry system in central China. The results indicated that the intercropping system, compared to a monocrop system containing only non-tree, crop species, increased the relative humidity by 6.9% to 8.4%, decreased surface soil temperatures by 1.7 to 1.9℃, reduced wind speed by 55% to 67%, reduced photosynthetic active radiation (PAR) by 13.9% to 24.5% and air CO2 concentration by 5.3-10.9μmol?mol-1. Changes in the net photosynthetic rate of soybean leaves at noon were primarily attributed to limitation of non stomatal factor. PAR and ambient air CO2 concentration were the dominant factors influencing net photosynthesis rate as shown by multiple stepwise regression analysis. The microclimatic of the intercropping system was the key factor preventing midday depression of photosynthesis that appeared in the monocropping system.