To obtain a large and thick AlN single crystal during sublimation growth, it is very important to maintain the thermal environment suitable for growth inside the crucible during a long period of time (>50 h). In this paper, an in-house developed integrated model capable of describing inductive, radiative and conductive heat transfer will be used to simulate the transient behavior of thermal environment inside the crucible during a 40-h experiment growth. Effects of graphite insulation degradation on temperature distribution inside the crucible will be investigated. Simulation results will be compared with the experimental data to study the effects of the insulation degradation-induced particle deposition, geometric variation of source material and crystal size enlargement on the temperature distribution in the crucible and the growth rate. The relationship between graphite insulation degradation and power input change of the induction-heated system will be established. The evolution of temperature difference between the source material and crystal, which is the driving force for growth, will be presented. This study will also provide the explanation of mechanism underling substantial reduction of growth rate after a long experiment run.