Abstract: By using the spatial replacement time and the typical sampling as m,taking Eucalyptus robust a plantation in Weiyuan County,Sichuan Province as materials,we comprehensively analyzed the composition of the understory vegetation species and important value,and the aboveground,underground and whole plant biomass of the shrub and herb layers and species diversity index(Shannon-Wiener diversity index H, Simpson dominance index H’,richness index D and Pielou evenness index J_SW),and explored the dynamic changes of species diversity and biomass of undergrowth vegetation at five different forest ages（4,5,6,7,8 years） and the correlation between them respectively.The results showed that:A total of 210 species were found,belonging to 79 families and 151 genera,with more species in the herb layer than in the shrub layer.The herb layer of Miscanthus.floridulus,Dicranopteris pedata and shrub layer of Melastoma malabathricum and Rubus hastifolius all occupied the dominant position at different ages respectively.With the forest age increasing,the density canopy increased,the D value,H value and H’ value of shrub layer all increased first and then decreased respectively;The D,H and H’ values of the herb layer showed a trend of change of the bimodal curve after increased,then decreased,then increased and then decreased.The biomass of the herb layer increased first and then decreased and then increased,the biomass of shrub layer increased first and then decreased.There was a significant positive correlation between species diversity and biomass,the D and H indices of herb layer were the direct factors affecting biomass.The species distribution,composition,species diversity and biomass of undergrowth vegetation in Eucalyptus robusta plantation were different in response to changes in forest age,showing different dynamic characteristic rules,the results provided data support for Eucalyptus robusta forest management in southwest China.