Abstract: Experiments were conducted in a automaticaly controled climat chamber to study the effect of barnyardgrass shading on soybean nitrogen fixation. Root interference between barnyardgrass and soybean was eliminated by seeding the crop in small pots which were put in big pots and the weed between the small and big pots. The results showed that barnyardgrass at density of 6, 12, and 24 plants/pot significantly reduced the light intensity over soybean plants averagely by 32.1, 39.1 and 43.4% leaf CO2 conductance by 14.0, 16.7 and 19.3%, leaf net phytosynthetic rate by 25.4, 32.7 and 37.3% during the growth season between 43 and 75 days after planting, and resulted in a marked decrease in N2-fixing rate per soybean plant by 31.5, 39.5 and 44.5%, respectively.