Abstract: According to the crown growth characteristics in industrial Eucalyptus stand of short rotation, the unit step function model was put forward for the process with changing point of short rotation Eucalyptus clones U6 based on the survey practical stands. The biology signification of the parameter of the model was analyzed by solving the coefficient. The analyzable conclusions of the regression model between crown diameter and age show that the utility of unit step function model in the crown growth process with the changing point is better than the usual regression methods, and the unit step function model can solve discontinuance on the changing point. The optimizing model results show that the power model is the best one in all models tested.