Abstract: According to field investigation data in Dongzhai Harbour of Hainan Island, the intraspecific and interspecific competitions of exotic species Sonneratia apetala (introduced in 1985) mixed forest were studied by using Hegyi’s competition index model for individual tree. The results show that the intraspecific competition intensity of S. apetala is positivelv correlated with the diameter scale of the trees, but beyond 20 cm in diameter the increment reduces obviously. The order of competition is: S. apetalaS. caseolarisAvicenia marinaKandelia candelRhizophora stylosaAegiceras corniculatum. The relationship between competition intensity and individual size of tree in the upper strata of trees followes closely the equation CI=ADB. The ecological effects of S. apetala are discussed. The authors consider that S. apetala can not make any ecological invasion in Dongzhai Harbour at present, but its effects in the future still need to be monitored.