To address engineering challenges encountered in developing advanced gas turbine combustors, theoretical analysis and numerical simulation were performed to investigate the thermoacoustic instability of a laminar lean premixed flame under autoignitive conditions. The analysis was carried out within the scope of weak autoignition tendencies and employed a three-step mechanism, which included the chaininitiation reaction. The numerical simulation was performed as a validation using a skeletal mechanism for diesel fuel under the typical operating conditions of gas turbines. The results showed that the threestep mechanism could qualitatively describe the high-temperature chemistry involved in autoignition and flame propagation. The analysis indicated that the flame propagation speed was accelerated by the accumulation of pre-flame reactions. Under certain conditions, the flame speed showed a linear correlation with the pre-flame length. The results were consistent with the existing literature. In addition, the equilibrium position of the flame oscillation, which was excited by velocity fluctuation, could return to the initial position, and pre-flame reactions could alter the phase delay. These results suggested a new mechanism for thermoacoustic instability because the overall heat release rate fluctuated with the flame front. The flame might amplify, damp, high-pass filter, or low-pass filter the acoustic energy based on the phase difference between the velocity and pressure oscillations. (C) 2020 The Combustion Institute. Published by Elsevier Inc. All rights reserved.