As an important component of global environmental change, the invasion of exotic species represents one of the most serious threats to native biodiversity and ecosystem function worldwide and causes huge economic losses. Light is one of the most important limiting resources for plant growth in subtropical and tropical forests, wherein the plant abilities of capturing and utilizing light becomes a determinant for their establishment and fitness. In this study, we compared plant growth, biomass allocation and leaf-level traits in response to variations in light intensity in Mikania micrantha and Chromolaena odorata, which have invaded many habitats in South China causing significant damage. We hypothesized that the invasion potential of exotic species will be restricted by the effective optical radiation they can capture, and light intensity may play an important role in determining the community resistance to invasion. Our results showed that the two exotic species both could survive under 3%-60% of full solar irradiance and display a relatively flexible biomass allocation strategy. It indicated that they have acclimated to a wide range of variation in light resources, which supplies ecological foundation for their widely spread. Low light significantly decreased the biomass accumulation of the two exotic species and might contribute to the low invasibility in monsoon evergreen broadleaf forest (MEBF) to plant invasions. In addition, we found that high light resulted in smaller and thicker leaves (low SLA, LAR and LARMR) in M. micrantha, which is advantageous to maintain the water balance under high light. When growing under low light conditions, M. micrantha increased its SLA and LAR. Its leaves turned to bigger and thinner, which is helpful to capture more light in adapting to the decreasing light. By contrast, C. odorata demonstrated no significant change in its leave-level traits across the light treatments. Collectively, the vine M. micrantha had higher morphological plasticity than herb C. odorata in leave-level traits, and higher morphological plasticity is helpful for M. micrantha to search light resources more effectively in variant light environments. And our results support the assumption that light may play an important role in determining the community invasion resistance.