The types of vegetation and the changes of the biodiversity were studied by investigating the species diversity of rubber forest in different age class. There are 207 species of vascular plants which belong to 113 genera and 61 families. The dominant species is shade-enduring family Gramineae. The typical vegetation communities of undergrowth were defined by vegetation analyses: (1) Association Juvenile Sun-Shrub. (2) Association Eupatorium odoratum. (3) Association Imperata cylindraca. (4) Association shade-enduring Gramineae. (5) Association Piper sarmentosum Roxb. (6) Association Dicranopteris pedata. (7) Association. Pteridophyta. (8) Association Sun-Gramineae. The biodiversity in rubber forest was different due to the rainfall, the soil nutrient content and the age of the rubber forest. The more plentiful the rainfall was, the more abundant the biodiversity was. The biodiversity showed that there was a declining trend with the increase of rubber forest’s age. A co-relationship between plant diversity and soil nutrient content was studied with plant community species diverse index. The biodiversity has a remarkable positive relation with soil organic matter, total N content, and unremarkable relation with soil P (total phosphorus, effective phosphorus) and K (total potassium and effective potassium) content.