Abstract: By comparing theoretical acoustic eigenmodes with the experimentally measured distribution of the peak pressure ratio under Ma = 0 quiescent flow and Ma = 0.8 subsonic flow, this study reveals the evolution mechanism of thermoacoustic mode shape in a novel pressure-gain combustion method. The results show that under quiescent inflow, the measured shape aligns with the first-order eigenmode (quarter-wave) of a left-closed, right-open resonator. In contrast, with subsonic inflow, the shape axially shifts rightward and transitions into a modified mode equivalent to a one-fifth-wave, while maintaining the traveling-wave behavior. This contrast demonstrates that valve-induced pulsating inflow actively governs both the modal composition and spatial structure of the thermoacoustic response. Moreover, high-frequency oscillation propagation of pressure waves confirms that the closing air valve acts as a closed end, suppressing backflow and ensuring positive reflection of compression waves to enable quasi-detonation combustion. This work establishes a novel approach to actively modulate the unsteady combustion through the regulation of the inflow conditions and the valve flapping frequency.