Abstract: In this paper，according to the law of density effect of plantation．a new basal growth model was estab-lished based on improving the basal area growth model proposed by senior research fellow Tang shouzheng．Using the data collected from the sample plots of Chinese fir kernel growing area of Fuiian，the parameters of the new model were fitted．The precision test result of the model showed that the improved basal area model has higher precision than the former．It also inuvduced an approach to analyse tranrang effect by using the improved basal growth model．

Key words: Chinese fir plantation, Basal area, Mathematical mcdel, Modified simplex method