The operation stability of pump turbine has a great impact on the pumped storage units. When the flow rate decreases, unstable head variations occur. Unstable working conditions will lead to unsafe operation. In order to study the detailed characteristics of flow regime and pressure pulsation of pump-turbine under different operating conditions, the internal flow and pressure pulsation for a model pump-turbine under two different guide vane opening angles from low to large flow of flow rates were analyzed. As the guiding mechanism, the guide vanes had interference on the flow. The results of the investigation showed that under both low flow rate with small guide vane opening angle and large flow rate with large guide vane opening angle, the pressure pulsations were small. Pressure value fluctuations were also small and the flow was steady. On the contrary, the pressure pulsations were large under both low flow rate with large guide vane opening angle and large flow rate with small guide vane opening angle. However, it was affected by many low-frequency under low flow rate with large guide vane opening angle and large flow with small guide vane opening angle. The components of frequency were more complex. Pressure pulsation amplitude and head fluctuations were increased. The internal flow was disordered. The head also had obvious pulsation. All of the above resulted in the poor operation stability. It showed that the matching relationship between guide vane opening and flow affected the operation stability. All the results provided reliable reference for improving the stability of pump-turbine under off-design operations.