The gas-liquid swirl coaxial injector is one of the most important injection elements for liquid rocket engines. However， this kind of injector is likely to suffer from self-pulsation， which may induce combustion instability and could even seriously affect the development of engines. At present， the understanding of the unsteady flow process inside the injector during self-pulsation is still very limited. By means of experimental and numerical simulation， this paper systematically analyzes the movement and deformation trajectory of liquid sheet， the causes of self-pulsation and the transmission process of pressure disturbance inside the liquid-centered swirl coaxial injector. It is found that the pressure oscillation of the flow field inside the injector is caused by the periodic blockage of the liquid sheet in the recess chamber， and the pressure oscillation first occurs outside the liquid sheet in the recess chamber. After that， the pressure oscillation is transmitted upstream through the gas annular gap and the internal pressure swirl injector with a certain phase difference， resulting in the pressure oscillation of the annular gap gas and central gas core with the same frequency. When self-pulsation intensity is large enough， it will cause the pressure oscillation of the liquid manifold， and the oscillation frequency is consistent with the self-pulsation frequency.