Abstract: Large air heaters are prone to combustion instability, so that the heater cannot work at the expected operating conditions, even leading to failures. Because it is difficult to ob-tain accurate parameters inside the engine combustion chamber through experiments, numerical simulation is an important means to predict the unstable combustion process. In this paper, by decoupling the combustion-acoustic system inside the combustion chamber, the flame response to the acoustic disturbance and the acoustic system response to the heat source disturbance are calculated separately. The combustion response under the acoustic disturbance is characterized as the flame transfer function. Finally, the most unstable modal frequency in combustion chamber can be obtained. The results show that the modal frequency of the envisaged air heater is 1389.9 Hz and is stable in the acoustic mode. The prediction results match the test results, proving that the method has good prediction ability.