In China, large-scale mangrove afforestation has been performed over the last two decades using introduced species. The large areas occupied by the introduced Sonneratia apetala have generated some concerns of biological invasion. The aim of this study was to investigate the feasibility of planting native mangrove species under the canopy of S. apetala plantations and the best species to acclimate under low light conditions in the understory. A 360-day experiment was conducted with four dominant mangrove species in southern China (S. apetala, and three native species Kandelia obovata, Aegiceras corniculatum, and Bruguiera gymnorhiza) in four habitats (i.e., bare mudflat, two-year plantation, six-year plantation, and eight-year plantation) with different light irradiance conditions. Results showed that in terms of propagule germination and seedling survival, the low light irradiance condition impeded the early growth of S. apetala, but had less impact on B. gymnorhiza. With decrease in light irradiance, increments in stem height, basal area, and leaf area, and relative growth rate decreased the most for S. apetala and the least for B. gymnorhiza among the four species. In terms of the physiological response parameters of net photosynthesis rate, stomatal conductance, and transpiration rate, B. gymnorhiza exhibited more adaptability than S. apetala under dense canopies. With respect to the response of early growth of seedlings over 12 months, B. gymnorhiza was the most adaptable species to the understory low light condition, while S. apetala was the most adversely affected species. The information from this study should be useful for the introduction of native species into plantations of introduced mangrove species to improve their conservation value.