Phenotypic plasticity is an indicator of the responses of plant growth and development to environmental changes, which reflects the fitness of plant individuals under environmental stress. However, the driving mechanism of plant phenotypic plasticity is still controversial. To explore the impacts of environmental conditions and competition on the phenotypic plasticity of leaf traits in trees, we surveyed two secondary forests with different slope directions in the Ngawa (Aba) Tibetan and Qiang Autonomous Prefecture in western Sichuan, China and analyzed the difference in the relationship between competition intensity and phenotypic plasticity of 10 tree species leaf functional traits between shade and sunny slopes. Results show that:(1) water and nutrient availabilities were greater on shade slope than those on sunny slope. (2) Mean intraspecific and interspecific competitions of tree species on shade slope were higher than those on sunny slope; intraspecific competition intensity decreased with the increases of individual tree size on shade slope, but it increased along with the increase of individual size on sunny slope. (3) The leaf traits include specific leaf area (SLA), leaf carbon content (C), leaf nitrogen content (N), leaf phosphorus content (P), leaf kalium content (K), leaf carbon-phosphorus ratio (CPR), leaf carbon-nitrogen ratio (CNR), and leaf nitrogen-phosphorus ratio (NPR). Phenotypic plasticity of all eight measured leaf traits on shade slope was higher than on sunny slope. Phenotypic plasticity of leaf traits increased with the increase of individual tree size on shade slope, but decreased with the increase of individual size on sunny slope. These findings suggest that the environmental resources of community on shade slope were greater than on sunny slope. The growth of forest trees was less restricted by the availability of environmental resources on shade slope, where higher competition intensity led to the intensification of resource overlap, especially the high intraspecific competition intensity, ultimately resulted in the higher phenotypic plasticity of leaf traits on shade slope. Because of higher competition intensity, with the increase of individual size, trees need higher phenotypic plasticity to gain competitive advantage and obtain more resources to support growth. However, because resources were relatively poor, the limitation of the availability of environmental resources reduced to the lower phenotypic plasticity of leave traits on sunny slope. Due to the limitation of the availability of environmental resources, with the increase of individual size, trees were more likely to suffer resource constraints. Therefore, phenotypic plasticity of leaf traits on sunny slope decreased with the increase of tree individual size. To summary, our study demonstrated that leaf phenotypic plasticity was mainly affected by competition intensity on the shade slope, while it was mainly limited by the availability of environmental resources on the sunny slope.