Recently, many studies have focused on the possibility of restoring mangrove ecosystems by introducing fast-growing mangroves. However, methods for managing an exotic fast-growing species to restore mangrove ecosystems and at the same time preventing invasion by introduced species remains unclear. Sonneratia apetala Buch-Ham is one example of an exotic mangrove with both high ecological value and potential risk for invasion after introduction. To investigate the possibility of reducing the potential for invasion by altering light availability, we simulated different irradiances of S. apetala understory in the greenhouse. For each irradiance treatment, three levels of competition between S. apetala and native mangroves Aegiceras corniculatum (L.) were used: no competition, intraspecific competition and interspecific competition. Compared with A. corniculatum, S. apetala showed a significantly higher growth rate for both height and biomass accumulation under full irradiation. Compared to the full irradiation treatment, the shading treatment significantly reduced the height, total biomass and biomass allocation to leaves of S. apetala by 61.31, 71.0, and 76.2%, respectively, whereas the growth of A. corniculatum was not affected. The results suggested that lowering light availability could inhibit the growth of S. apetala and increase the competitiveness of A. corniculatum. Planting introduced fast-growing mangroves at a density of approximately 2,000 plants/hm2 is an effective strategy for preventing potential invasion and restoring wetland habitats. By taking advantage of the differences in shade tolerance between fast-growing exotic mangroves and native mangroves, introduction of fast-growing mangroves in coastal areas could have huge potential for reforesting mangrove ecosystems.