Based on the theories of statistical analysis and differential equation, two methods were given for estimating the transition probability in the diameter grade transition model of forest population. The first method was used for the estimation when two groups of observation data were given and it was no necessary to consider the environmental factors of forest stand, while the second one was used for that when the environmental factors were known and there was no need of the observation data. The results of case studies showed that both of the two methods had the characteristics of easy operation and high practicability, and the importance of theoretical guidance and practical application in forest management.