Abstract: In 2021, the Bruguiera gymnorhiza population in a fixed sample plot (1 hm²) located within the National Mangrove Protection Zone of Dongzhai Port, Hainan was selected as the research subject.The structural characteristics and dynamic changes of the population were analyzed and studied using ecological middle diameter class instead of age class methodology.The results showed that: ① the diameter class structure of the population in this area showed an spindle-type, and the sum of the number of individuals in the Ⅰ-Ⅳ age class accounted for 75.4% of the total number of individuals in the population, and the number of plant individuals decreased significantly after the Ⅳ age class.② The index of population dynamic change reflects that the population in this area is a growing population, but the population is unstable, and the population growth trend is not significant under external interference.③ The survival curve of the population in this area belongs to Deevey-Ⅲ type, which means that the mortality rate of the individuals in the younger stage of the population is relatively high, and the mortality rate of the individuals in the older stage tends to be stable.④ The static life table, mortality curve, disappearance rate curve and survival analysis of the population in this area reflect that the dynamic changes of the population in the younger stage are relatively drastic, and the dynamic changes of the population in the advanced stage are relatively stable.⑤ The time series analysis of the population size of the region also reflected that the population size of the low-age group declined gradually after 2 to 3 age groups, while the population size of the middle-age group increased first and then decreased during the population development process after 2 to 9 age groups.With the increase of age class time, the total number of individuals in the population showed a gradually decreasing trend.