Abstract: In order to investigate flow characteristics of the separation flow, cold flow tests were conducted in a solid rocket motor nozzle with a high expansion ratio under the sea level condition. The influences of the inlet pressure on the pressure of test points upstream and downstream the separation location were revealed in the tests and pressure data were analyzed by means of FFT. The results show that the pressure increases for test points upstream the separation location but decreases for test points downstream the separation location. The pressure is steady and right below the ambient pressure for test points downstream and far away from the separation location. Pressure fluctuations become more evident after separation in comparison with that before separation, and the enhanced pressure fluctuations were mainly low-frequency fluctuations in the range of 50Hz. The research has important reference to the designing and testing of the nozzle with a high expansion ratio.