Growth and biomass allocation of differently-aged populations of Spartina alterniflora were studied by methodology of “spatial for temporal gradient”. The results showed that: the height growth of Spartina alterniflora populations presented a downward trend with the increase of population age, and the height growth of 6 and 16 years old populations were significantly lower than that of 2 years old population (P<0.05); the density, total biomass and above-ground biomass of populations decreased with the increase of population age, with significant difference between those of 16 and 2 years old populations (P<0.05); the below-ground biomass and root-shoot ratio increased in the first few years followed with reducing thenafter, and the below-ground biomass of 16 years old population was significantly lower than that of 2 years old population (P<0.05); the assimilative organic biomass didn′t change with population age, while non-assimilative organic biomass reduced with increase of population age with those of 6 and 16 years old populations being significantly lower than that of 2 years old population (P<0.05); the fibrous roots and rhizome biomass increased in the first few years followed with reducing. All the results indicated that the 16 years old population of Spartina alterniflora presented a declining trend and more understanding of the declining reasons will be helpful to the management and control of Spartina alterniflora in China.