The ecosystem service values and functions of forests are important in the semi-arid region of eastern China. With the increasing of climate change and the degradation of the aridification, forest declines have been monitored in the region in recent years. It eventually changed the species composition, nutrient cycling and biomass accumulation cycle of the ecosystem, and then affected the forest regeneration and structure, caused disturbance to the service function and biodiversity of the forest ecosystem and to a certain extent threatened the human society, culture and ecological function, and promoted the sustainable development of social production and environmental protection. Population competition is an intrinsic driving factor of forest dynamics. But there is still lack of evidences on the relationships between population competitions and forest decline in the area. Using typical forest types in Daxing'anling, the Inner Mongolia as cases, a suitable competition index model was used to analyze the competition status at both individual and population levels; then individual tree chronology (TRI) was selected as an indicator of individual growth reduction; Plot chronology and basal area increment (BAI) were applied to assess forest decline at the plot level. After that, the relationships between the competition indices and the forest decline indicators at different levels were analyzed. Results indicated that the chronology of the plot was basically same as the decay period indicated by the individual chronology of the tree. The decay period of each plot could be concluded by combining the coincidence results. There was also a coincidence between periods of growth decline in different plots. At the individual chronology, most plots had similar periods of 2000-2005, with serious decline over the threshold defined by that the percentage of trees with growth decline exceeding 50%. At the plot chronology, plots L-WCG1, L-WCG2, and L-WRG had the same period of growth decline from 1989 to 1997, and plots L-HDG, L-WCG1, L-WCG2, L-WRG and L-GH1 had the same periods of growth decline from 1998 to 2003. The reason may be that the serious drought events appeared in the stage, causing the growth of trees to be inhibited at a large scale. The individual five-year averagely relative basal area increment (rBAI₅) in each plot had the highest correlation with the individual competition index. The relationship between the two can be expressed by an exponential function equation through trend fitting. That is, rBAI₅ decreased as the individual level competition index increased. Thus the individual competition index had a significant impact on the growth of trees in five years. The relationships between the competition index at plot level and the mean values of the basal area increment in recent two, five and ten years (BAI₂, BAI₅ and BAI₁₀) were insignificant. The research explored the factors of forest decline in semi-arid area of eastern China from the perspective of population competition. It aims at providing bases for the analyses of the driving mechanism of forest decline and the sustainable development of forest resources in semi-arid area.