Abstract: Study on the simulation of diameter growth of Chinese Fir is conducted based on the competitive mechanism of stands. The crown and its length are taken as main competitive indexes, and the competitive function f(Cr,l) as main growth potentials to calculate potential growth function fp(Cr,l) and practical growth function fd(Cr,l) of stands. The curve of potential diameter growth of a stand can be therefore brought out from the practical growth function adjusted by coefficient K. It is considered that the potential diameter growth of a stand is the growth of diameter under the environment without any competition, namely the potentials of diameter growth of a stands is fully played. So diameter growth models can be calculated through the potential growth function adjusted by competitive coefficient C. Furthermore, the forecasting model of diameter growth can be gotten from the comparison of competitive potentials f(Cr,l). The model can be used to forecast the annual growth of stands, and provide guidance for thinning and management of stands. It is also useful for the estimation of rational rotation and comprehensive management of stands.