Abstract: Interspecific relationship is an important biological basis for high-yield and efficient utilization of resources in intercropping systems. It can reveal the effects of interspecific relationships on the yield of intercropping systems, providing the theoretical and guiding basis for optimizing intercropping agriculture. To this end, a pot experiment was set up to investigate the effects of root interaction and planting density on grain yield, interspecific competition and complementarity of barley-pea intercropping system. In the experiment, two kinds of species interactions were set, only aboveground interaction (whole root partition of two intercropped crops) and aboveground and belowground interaction (without root partition between two intercropped crops), in combination with two planting densities of barley (15 plantspot^-1 and 20 plantspot^-1). The study was designed to provide the theoretical basis for raising yield management technique under intercropping based on interspecific relationship optimization. The study showed that: 1) compared with the corresponding monoculture system, average dry matter accumulation of intercropping systems increased by 3.6%–11.3%. The contribution rate of belowground interaction was 53.9%–63.5%. For the intercropping system with belowground and aboveground interactions, increased planting density of barley enhanced dry matter accumulation of the system by 12.5%–14.4%. But, for the intercropping system only with aboveground interaction, it increased by 3.3%–6.7%. Similarly, barley-pea intercropping total grain yield was 8.6%–33.8% higher than the average grain yield of the corresponding monocultures. The contribution rate of the existing belowground interaction was 2.4%–16.2%. With increasing planting density of barley, the intercropping system existing both belowground and aboveground interactions increased total grain yield by 7.0%–10.9%; while the intercropping system existing only aboveground interaction only increased by 1.2%–2.6%. This suggested that belowground interaction was critical for close planting in intercropping systems. 2) Compared with corresponding monoculture treatments, intercropping increased harvest indexes (i.e., HI) of barley and pea by 8.7%–21.0% and 3.3%–31.7%, respectively. The harvest index of intercropped barley increased with increasing planting density of barley while that of intercropped pea decreased. The trend of the decline was more obvious under whole root partition. 3) Belowground interaction without root partition increased land equivalent ratio (LER), but high planting density of barley decreased LER in barley-pea intercropping system. This suggested that belowground root interaction was the main force behind the advantages of intercropping. 4) The belowground interaction significantly increased the competitiveness (by up to 40.1%–89.1%) of barley to pea during the period of co-growth. The average competitiveness increased by 11.0%–49.9% with increasing barley planting density. 5) A quadratic relationship existed between total grain yield of barley-pea intercropping system and the average competitiveness of barley to pea during the whole period of co-growth. High grain yields of both crops were obtained when competitiveness was 0.13–0.33. Our results showed that appropriately improving the competitiveness of barley and pea intercropping system (especially at barley grain-filling stage) by increasing the planting density of barley (25 plantpot^-1) increased total grain yield of the intercropping system.