Abstract: In recent years,the study on forest vegetation growth has made some progress.It is also a basis of the computer model of FVS,which is U.S.forest vegetation simulation software.In this paper FVS is also reference prototype to Chinese forest vegetation system.In the research of finding of suitable growth model to our country's forest,the study shows that the biological activity under the cover of various ecological factors and vegetation growth becomes extremely complex.Which model should the study begin with? How to choose the base factor or base model for forest vegetation? How to work out a set of single-tree-growth prediction system applied to various types of vegetation in our country? These questions become difficulties in the research.In the study there were lots of inventory data of Pinus tabulaeformi and Platycladus orientalis in Beijing area.A lot of various growth models of FVS have been researched,reused and remodified.The calculation results showed that calculation of DBH played a decisive role in the forecast of forest vegetation growth.And it can be asserted that DBH is a key factor and important datum,which can play a breakthrough role in the study of all forest vegetation simulation system.

Key words: FVS, diameter at breast height, growth of stand, forest vegetation