<p>The lean premixed combustion mode makes gas turbine combustors prone to thermoacoustic oscillations, posing a significant threat to the safe operation of gas turbines. Frequency and amplitude are the key parameters of thermoacoustic oscillations, and they vary with the change of equivalence ratio. The transition of these oscillation modes is closely related to the acoustic boundary conditions of the system. This paper investigates the effect of equivalence ratio changes on acoustic boundary conditions. In the experiments, the system was induced to oscillate in different modes by altering the equivalence ratio, and the variation of the acoustic reflection coefficient and acoustic impedance at the outlet with equivalence ratio was analyzed using two-pressure-transducer method. Finite element numerical simulation was used to study the effect of acoustic boundary condition changes on the growth rate of oscillation modes. The results show that there are three main oscillation modes. As the combustor equivalence ratio increases, the oscillation mode shifts from low frequency to high frequency. There are significant differences in the outlet acoustic reflection coefficient and acoustic impedance among the three modes, especially in phase, with a maximum difference of up to 180°. The finite element numerical simulation results indicate that changes in boundary conditions are key factors affecting the growth rate of modes, and the oscillation modes observed in the experiments correspond to those with the highest growth rate, with a frequency error within 1.94%. The above results indicate that changes in equivalence ratio can lead to variations in acoustic boundary conditions, thereby causing a shift in oscillation modes.</p>