Abstract: The aim of this study was to provide reference and theoretical basis of mixed forest afforestation and close-to-nature silvicultural system. Relationship of tree species in close-to-nature operation was revealed by analyzing intra- and inter-species competition intensity. We took 20 permanent sample plots of 5 kinds of close-to-nature transformation forests as research object in the Center of Tropical Forestry which is sited at Pingxiang County of Guangxi Province (A1-A4: mixed forest with different cutting intensities and enrichment plantations in operation, A5: monoculture forest in control, 4 permanent sample plots in each operation model). Tree growth space and neighbors were determined by using Voronoi diagram. Heygi individual competition indices (CI) were used to analyze the competition of mingled forest, intraspecific and interspecific competition of Pinus massoniana in mingled and monoculture forest. The results showed that 1) the competition of P. massoniana and the main planting species in mixed forest were ranked as: Castanopsis hystrix Manglietia glauca Quercus griffithii Pinus massoniana Michelia hedyosperma Erythrophleum fordii, and average competition index in different transformation modes was ranked as: A1 A3 A4 A2. 2) The number of objective and competitive trees was 212 and 336, respectively in mixed stand. The number of P. massoniana was 190 in monoculture stand. The competition index in monoculture forest (359.51) was larger than in mixed forest (166.98). It meant that the appropriate intensity of logging effectively reduced the pressure of competition. The intraspecific competition only existed in monoculture forest. The intraspecific and interspecific competition index of P. massoniana was each accounted for 54.65% and 45.35% in the mixed forests. 3) The intraspecific competition index of Pinus massoniana had significant difference among mixed forest and monoculture forest. It decreased gradually with the increase of diameter at breast height (DBH), and the average competition indices with diameter grade in mixed forest were less than in control monoculture forest. 4) The interspecific competition pressure came from fast-growing trees firstly. The relationship of Pinus massoniana and main species (CI1) followed as: Quercus griffithii Manglietia glauca Castanopsis hystrix Michelia hedyosperma Schefflera octophylla Trema cannabina Styrax tonkinensis Erythrophleum fordii Ficus esquiroliana. The interspecific competition between each model had no difference. 5) The index of Pinus massoniana decreased with the increase of DBH. The relationship between total and intraspecific competition index and DBH accorded with power and hyperbola function respectively in mixed forest. The relationship between competition index and DBH conformed to logarithmic function in monoculture forest. It showed a stronger competition intensity at DBH of 25 cm and appeared weaker at DBH over 25 cm. According to analysis of competition of close-to-nature transformation forest, we know that cutting disturbance trees and enriching species could reduce competitive pressure of Pinus massoniana and facilitate growth of enrichment species and natural regeneration. As well as it could guide positive succession to form multiple layer, uneven-aged and mixed forest. This study proved the rationality and feasibility of transformation from monoculture plantation to close-to-nature forest.