Abstract: To investigate the mechanism of high planting density in cereal-legume intercropping system, a pot experiment of barley-pea intercropping system was carried out and the effects of nitrogen (N) application and root barrier on nitrogen and fertilizer use efficiency under high planting density of barley were determined. In the experiment, three factors, each with two treatment levels-N fertilizer applicationno N application and N application with 100 mg(N)·kg^-1, root barrierno barrier with root interaction and root barrier without root interaction) and planting density (low density with 15 barley plants per pot and high density with 25 barley plants per pot) were set up. The results showed that:1) N application, plant root interaction and high barley planting density improved N uptake of barley-pea intercropping system. Compared with no N application treatment, N uptake increased by 33.8% in N application treatment. There was also 81.1% increase in N uptake under no root barrier treatment over root barrier treatment. N uptake under high planting density treatment increased by 4.2% compared with low planting density treatment. Plant root interaction improved N uptake by 92.4% and 11.0%, respectively, under no N application and N application treatments. Increasing planting density with root interaction significantly increased N uptake of the intercropping system. 2) Barely plant performed better for N competition, and its' N competition ratio significantly increased under high planting density. However, N application reduced barley N competition ratio. Compared with pea, barley was highest in competitive advantage at heading stage. 3) Root interaction improved grain N content of barley and pea, respectively, by 126.7% and 26.9% under N application treatment. Also barley and pea kernel N content increased, respectively, by 188.5% and 46.5% under no N treatment. There was a significant interaction between N application and root interaction for kernel N content. 4) High barley planting density significantly improved N use efficiency by 59.8% under root interaction treatment of the intercropping system. N competition between barley and pea was positively correlated with N use efficiency in the intercropping plant population. In conclusion, interactions of N application, root barrier and barley planting density enhanced crop productivity of barley-pea intercropping system. Proper N application and sufficient root interaction made feasible high planting density intercropping system, optimized competition between barley and pea, and improved N uptake and use efficiency.