The pressure oscillation in the bulk-load liquid gun is a kind of combustion instability and has adverse effects on the interior ballistic cycle. With different liquid propellants and different combustion chamber geometries, the combustion and propulsion experiments were conducted on a small calibre gun, and the behaviors of pressure oscillation were investigated. By the wavelet transform, the pressure oscillations were precisely extracted from the measured pressure-time curves. The oscillation characteristics were analyzed and compared in both the time domain and the frequency domain. Furthermore, a simplified acoustic resonance model considering the combustion-gas cavity expansion in liquid propellant was developed to estimate the range of the resonant frequency, and the relations between pressure oscillation and acoustic resonance were discussed. The experiments showed that the stepped-wall combustion chamber geometry was beneficial to reduce the pressure oscillation. The consistency of frequency range of the low-frequency oscillation between calculations and experiments suggested that axial acoustic resonance played a key role in producing the pressure oscillation, and the characteristics of the high-frequency oscillation were mainly affected by the combustion of liquid propellant droplets.