To clarify the transient flow characteristics of a high-speed aero-fuel centrifugal pump in variable gas-liquid ratio conditions, numerical simulations for the internal flow field in design flow rate and small flow rate conditions are conducted, focusing on the transient flow characteristics and time-frequency performance of pressure pulsation in the impeller channel. The conversion relationship between gas-liquid ratio and inlet pressure is given to determine the inlet simulation boundary, and then the grid model and length of time step are checked for relevant test. The prediction results between simulations and test are given to verify the effectiveness of the adopted simulation method. Then, the transient characteristics are analyzed through the results of pressure contour and turbulent kinetic energy, and the time-frequency performances of pressure pulsations at impeller inlet and outlet are conducted by fast Fourier transform(FFT). The results show that the flow in the impeller channel is relatively stable under the fuel saturation condition, and the main frequency of pressure amplitude is rotation frequency. With the increase of gas-liquid ratio, the impeller inlet produces a low-pressure zone whose area is significantly enlarged. Besides, a certain wake flow zone is generated at impeller outlet, where the turbulent energy dissipation rate is also demonstrated to be the strongest at these zones. Moreover, the inlet pressure is generally decreased with the increase of gas-liquid ratio, and the main frequency at the design flow rate is rotation frequency, but other frequency multiplication appears at the small flow rate. Meanwhile, the wake flow at the impeller outlet does not seriously affect the main frequencies at the monitoring points, where the main frequency is still rotation frequency.